Bug 1438687 - Add Developer documentation for LocaleService. r=jfkthame,Pike
authorZibi Braniecki <zbraniecki@mozilla.com>
Thu, 26 Apr 2018 14:18:34 -0700
changeset 801195 38922a89e9d5d957a95e4344a369a5ef345c5a1c
parent 801194 56bf1eeea50a4dfb26864fc983ef32ea410d1147
child 801196 e9b7d0f51d867108e47f587019718958047b3320
push id111603
push usermozilla@kaply.com
push dateTue, 29 May 2018 22:12:07 +0000
reviewersjfkthame, Pike
bugs1438687
milestone62.0a1
Bug 1438687 - Add Developer documentation for LocaleService. r=jfkthame,Pike MozReview-Commit-ID: Jk2BhKBsW8S
intl/docs/index.rst
intl/docs/locale.rst
intl/docs/locale_env.rst
intl/docs/locale_startup.rst
intl/moz.build
new file mode 100644
--- /dev/null
+++ b/intl/docs/index.rst
@@ -0,0 +1,26 @@
+====================
+Internationalization
+====================
+
+Internationalization (`"i18n"`) is a domain of computer science focused on making
+software accessible across languages, regions and cultures.
+A combination of those is called a `locale`.
+
+On the most abstract level, Gecko internationalizaion is a set of algorithms,
+data structures and APIs that aim to enable Gecko to work with all human scripts and
+languages, both as a UI toolkit and as a web engine.
+
+In order to achieve that, i18n has to hook into many components such as layout, gfx, dom,
+widget, build, front-end, JS engine and accessiblity.
+It also has to be available across programming languages and frameworks used in the
+platform and front-end.
+
+Below is a list of articles that introduce the concepts necessary to understand and
+use Mozilla's I18n APIs.
+
+.. toctree::
+   :maxdepth: 1
+
+   locale
+   dataintl
+   localization
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+++ b/intl/docs/locale.rst
@@ -0,0 +1,585 @@
+.. role:: js(code)
+   :language: javascript
+
+=================
+Locale management
+=================
+
+A locale is a combination of language, region, script, and regional preferences the
+user wants to format their data into.
+
+There are multiple models of locale data structures in the industry that have varying degrees
+of compatibility between each other. Historically, each major platform has used their own,
+and many standard bodies provided conflicting proposals.
+
+Mozilla, alongside with most modern platforms, follows Unicode and W3C recommendation
+and conforms to a standard known as `BCP 47`_ which describes a low level textual
+representation of a locale known as `language tag`.
+
+A few examples of language tags: *en-US*, *de*, *ar*, *zh-Hans*, *es-CL*.
+
+Locales and Language Tags
+=========================
+
+Locale data structure consists of four primary fields.
+
+ - Language (Example: English - *en*, French - *fr*, Serbian - *sr*)
+ - Script (Example: Latin - *Latn*, Cyrylic - *Cyrl*)
+ - Region (Example: United States - *US*, Canada - *CA*, Russia - *RU*)
+ - Variants (Example: Mac OS - *macos*, Windows - *windows*, Linux - *linux*)
+
+`BCP 47`_ specifies the syntax for each of those fields (called subtags) when
+represented as a string. The syntax defines the allowed selection of characters,
+their capitalization and order in which the fields should be defined.
+
+Most of the base subtags are valid ISO codes, such as `ISO 639`_ for
+language subtag, or `ISO 3166-1`_ for region.
+
+The examples above present language tags with several fields omitted, which is allowed
+by the standard.
+
+On top of that a locale may contain:
+
+ - extensions and private fields
+     These fields can be used to carry additional information about a locale.
+     Mozilla currently has partial support for them in the JS implementation and plans to
+     extend support to all APIs.
+ - extkeys and grandfathered tags
+     Mozilla does not support these yet.
+
+
+An example locale can be visualized as:
+
+.. code-block:: javascript
+
+  {
+      "language": "sr",
+      "script": "Cyrl",
+      "region": "RU",
+      "variants": [],
+      "extensions": {},
+      "privateuse": [],
+  }
+
+which can be then serialized into a string: **"sr-Cyrl-RU"**.
+
+.. important::
+
+  Since locales are often stored and passed around the codebase as
+  language tag strings, it is important to always use an appropriate
+  API to parse, manipulate and serialize them.
+  Avoid `Do-It-Yourself` solutions which leave your code fragile and may
+  break on unexpected language tag structures.
+
+Locale Fallback Chains
+======================
+
+Locale sensitive operations are always considered "best-effort". That means that it
+cannot be assumed that a perfect match will exist between what the user requested and what
+the API can provide.
+
+As a result, the best practice is to *always* operate on locale fallback chains -
+ordered lists of locales according to the user preference.
+
+An example of a locale fallback chain may be: :js:`["es-CL", "es-ES", "es", "fr", "en"]`.
+
+The above means a request to format the data according to the Chilean Spanish if possible,
+fall back to Spanish Spanish, then any (generic) Spanish, French and eventually to
+English.
+
+.. important::
+
+  It is *always* better to use a locale fallback chain over a single locale.
+  In case there's only one locale available, a list with one element will work
+  while allowing for future extensions without a costly refactor.
+
+Language Negotiation
+====================
+
+Due to the imperfections in data matching, all operations on locales should always
+use a language negotiation algorithm to resolve the best available set of locales
+based on the list of all available locales and an ordered list of requested locales.
+
+Such algorithms may vary in sophistication and number of used strategies. Mozilla's
+solution is based on modified logic from `RFC 5656`_.
+
+The three lists of locales used in negotiation:
+
+ - **Available** - locales that are locally installed
+ - **Requested** - locales that the user selected in decreasing order of preference
+ - **Resolved** - result of the negotiation
+
+The result of a negotiation is an ordered list of locales that are available to the system
+and it is expected for the consumer to attempt using the locales in the resolved order.
+
+Negotiation should be used in all scenarios like selecting language resources,
+calendar, number formatting, etc.
+
+Single Locale Matching
+----------------------
+
+Every negotiation strategy goes through a list of steps in an attempt to find the
+best possible match between locales.
+
+The exact algorithm is custom, and consists of a 6 level strategy:
+
+::
+
+  1) Attempt to find an exact match for each requested locale in available
+     locales.
+     Example: ['en-US'] * ['en-US'] = ['en-US']
+
+  2) Attempt to match a requested locale to an available locale treated
+     as a locale range.
+     Example: ['en-US'] * ['en'] = ['en']
+                            ^^
+                            |-- becomes 'en-*-*-*'
+
+  3) Attempt to use the maximized version of the requested locale, to
+     find the best match in available locales.
+     Example: ['en'] * ['en-GB', 'en-US'] = ['en-US']
+                ^^
+                |-- ICU likelySubtags expands it to 'en-Latn-US'
+
+  4) Attempt to look for a different variant of the same locale.
+     Example: ['ja-JP-win'] * ['ja-JP-mac'] = ['ja-JP-mac']
+                ^^^^^^^^^
+                |----------- replace variant with range: 'ja-JP-*'
+
+  5) Attempt to look for a maximized version of the requested locale,
+     stripped of the region code.
+     Example: ['en-CA'] * ['en-ZA', 'en-US'] = ['en-US', 'en-ZA']
+                ^^^^^
+                |----------- look for likelySubtag of 'en': 'en-Latn-US'
+
+  6) Attempt to look for a different region of the same locale.
+     Example: ['en-GB'] * ['en-AU'] = ['en-AU']
+                ^^^^^
+                |----- replace region with range: 'en-*'
+
+Filtering / Matching / Lookup
+-----------------------------
+
+When negotiating between lists of locales, Mozilla's :js:`LocaleService` API
+offers three language negotiation strategies:
+
+Filtering
+^^^^^^^^^
+
+This is the most common scenario, where there is a benefit to creating a maximal
+possible list of locales that the user may benefit from.
+
+An example of a scenario:
+
+.. code-block:: javascript
+
+    let requested = ["fr-CA", "en-US"];
+    let available = ["en-GB", "it", "en-ZA", "fr", "de-DE", "fr-CA", "fr-CH"];
+
+    let result = Services.locale.negotiateLanguages(requested, available);
+
+    result == ["fr-CA", "fr", "fr-CH", "en-GB", "en-ZA"];
+
+In the example above the algorithm was able to match *"fr-CA"* as a perfect match,
+but then was able to find other matches as well - a generic French is a very
+good match, and Swiss French is also very close to the top requested language.
+
+In case of the second of the requested locales, unfortunately American English
+is not available, but British English and South African English are.
+
+The algorithm is greedy and attempts to match as many locales
+as possible. This is usually what the developer wants.
+
+Matching
+^^^^^^^^
+
+In less common scenarios the code needs to match a single, best available locale for
+each of the requested locales.
+
+An example of this scenario:
+
+.. code-block:: javascript
+
+    let requested = ["fr-CA", "en-US"];
+    let available = ["en-GB", "it", "en-ZA", "fr", "de-DE", "fr-CA", "fr-ZH"];
+
+    let result = Services.locale.negotiateLanguages(
+      requested,
+      available,
+      undefined,
+      Services.locale.langNegStrategyMatching);
+
+    result == ["fr-CA", "en-GB"];
+
+The best available locales for *"fr-CA"* is a perfect match, and for *"en-US"*, the
+algorithm selected British English.
+
+Lookup
+^^^^^^
+
+The third strategy should be used in cases where no matter what, only one locale
+can be every used. Some third-party APIs don't support fallback and it doesn't make
+sense to continue resolving after finding the first locale.
+
+It is still advised to continue using this API as a fallback chain list, just in
+this case with a single element.
+
+.. code-block:: javascript
+
+    let requested = ["fr-CA", "en-US"];
+    let available = ["en-GB", "it", "en-ZA", "fr", "de-DE", "fr-CA", "fr-ZH"];
+
+    let result = Services.locale.negotiateLanguages(
+      requested,
+      available,
+      Services.locale.defaultLocale,
+      Services.locale.langNegStrategyLookup);
+
+    result == ["fr-CA"];
+
+Default Locale
+--------------
+
+Besides *Available*, *Requested* and *Resolved* locale lists, there's also a concept
+of *DefaultLocale* which is a single locale out of the list of available ones that
+should be used in case there is no match to be found between available and
+requested locales.
+
+Every Firefox is built with a single default locale - for example
+**Firefox zh-CN** has *DefaultLocale* set to *zh-CN* since this locale is guaranteed
+to be packaged in, have all the resources, and should be used if the negotiation fails
+to return any matches.
+
+.. code-block:: javascript
+
+    let requested = ["fr-CA", "en-US"];
+    let available = ["it", "de", "zh-CN", "pl", "sr-RU"];
+    let defaultLocale = "zh-CN";
+
+    let result = Services.locale.negotiateLanguages(requested, available, defaultLocale);
+
+    result == ["zh-CN"];
+
+Chained Language Negotiation
+----------------------------
+
+In some cases the user may want to link a language selection to another component.
+
+For example, a Firefox extension may come with its own list of available locales, which
+may have locales that Firefox doesn't.
+
+In that case, negotiation between user requested locales and the addon's list may result
+in a selection of locales superseding that of Firefox itself.
+
+
+.. code-block:: none
+
+         Fx Available
+        +-------------+
+        |  it, fr, ar |
+        +-------------+                 Fx Locales
+                      |                +--------+
+                      +--------------> | fr, ar |
+                      |                +--------+
+            Requested |
+     +----------------+
+     | es, fr, pl, ar |
+     +----------------+                 Addon Locales
+                      |                +------------+
+                      +--------------> | es, fr, ar |
+      Addon Available |                +------------+
+    +-----------------+
+    |  de, es, fr, ar |
+    +-----------------+
+
+
+In that case, an addon may end up being displayed in Spanish, while Firefox UI will
+use French. In most cases this is a bad UX.
+
+In order to avoid that, one can chain the addon negotiation and take Firefox's resolved
+locales as a `requested` and negotiate that against the addons' `available` list.
+
+.. code-block:: none
+
+        Fx Available
+       +-------------+
+       |  it, ar, fr |
+       +-------------+                Fx Locales (as Addon Requested)
+                     |                +--------+
+                     +--------------> | fr, ar |
+                     |                +--------+
+           Requested |                         |                Addon Locales
+    +----------------+                         |                +--------+
+    | es, fr, pl, ar |                         +------------->  | fr, ar |
+    +----------------+                         |                +--------+
+                                               |
+                               Addon Available |
+                             +-----------------+
+                             |  de, es, ar, fr |
+                             +-----------------+
+
+Available Locales
+=================
+
+In Gecko, available locales come from the `Packaged Locales` and the installed
+`language packs`. Language packs are a variant of web extensions providing just
+localized resources for one or more languages.
+
+The primary notion of which locales are available is based on which locales Gecko has
+UI localization resources for, and other datasets such as internationalization may
+carry different lists of available locales.
+
+Requested Locales
+=================
+
+The list of requested locales can be read using :js:`LocaleService::GetRequestedLocales` API,
+and set using :js:`LocaleService::SetRequestedLocales` API.
+
+Using the API will perform necessary sanity checks and canonicalize the values.
+
+After the sanitization, the value will be stored in a pref :js:`intl.locale.requested`.
+The pref usually will store a comma separated list of valid BCP47 locale
+codes, but it can also have two special meanings:
+
+ - If the pref is not set at all, Gecko will use the default locale as the requested one.
+ - If the pref is set to an empty string, Gecko will look into OS app locales as the requested.
+
+The former is the current default setting for Firefox Desktop, and the latter is the
+default setting for Firefox for Android.
+
+If the developer wants to programmatically request the app to follow OS locales,
+they can call the :js:`SetRequestedLocales` API with no argument.
+
+Regional Preferences
+====================
+
+Every locale comes with a set of default preferences that are specific to a culture
+and region. This contains preferences such as calendar system, way to display
+time (24h vs 12h clock), which day the week starts on, which days constitute a weekend,
+what numbering system and date time formatting a given locale uses
+(for example "MM/DD" in en-US vs "DD/MM" in en-AU).
+
+For all such preferences Gecko has a list of default settings for every region,
+but there's also a degree of customization every user may want to make.
+
+All major operating systems have a Settings UI for selecting those preferences,
+and since Firefox does not provide its own, Gecko looks into the OS for them.
+
+A special API :js:`mozilla::intl::OSPreferences` handles communication with the
+host operating system, retrieving regional preferences and altering
+internationalization formatting with user preferences.
+
+One thing to notice is that the boundary between regional preferences and language
+selection is not strong. In many cases the internationalization formats
+will contain language specific terms and literals. For example a date formatting
+pattern into Japanese may look like this - *"2018年3月24日"*, or the date format
+may contains names of months or weekdays to be translated
+("April", "Tuesday" etc.).
+
+For that reason it is tricky to follow regional preferences in a scenario where Operating
+System locale selection does not match the Firefox UI locales.
+
+Such behavior might lead to a UI case like "Today is 24 października" in an English Firefox
+with Polish date formats.
+
+For that reason, by default, Gecko will *only* look into OS Preferences if the *language*
+portion of the locale of the OS and Firefox match.
+That means that if Windows is in "**en**-AU" and Firefox is in "**en**-US" Gecko will look
+into Windows Regional Preferences, but if Windows is in "**de**-CH" and Firefox
+is in "**fr**-FR" it won't.
+In order to force Gecko to look into OS preferences irrelevant of the language match,
+set the flag :js:`intl.locale.use_os_preferences` to :js:`true`.
+
+UI Direction
+------------
+
+Since the UI direction is so tightly coupled with the locale selection, the
+main method of testing the directionality of the Gecko app lives in LocaleService.
+
+:js:`LocaleService::IsAppLocaleRTL` returns a boolean indicating the current
+direction of the app UI.
+
+Default and Last Fallback Locales
+=================================
+
+Every Gecko application is built with a single locale as the default one. Such locale
+is guaranteed to have all linguistic resources available, should be used
+as the default locale in case language negotiation cannot find any match, and also
+as the last locale to look for in a fallback chain.
+
+If all else fails, Gecko also support a notion of last fallback locale, which is
+currently hardcoded to *"en-US"* and is the very final locale to try in case
+nothing else (including the default locale) works.
+Notice that Unicode and ICU use *"en-GB"* in that role because more English speaking
+people around the World recognize British regional preferences than American (metric vs.
+imperial, fahrenheit vs celsius etc.).
+Mozilla may switch to *"en-GB"* in the future.
+
+Packaged Locales
+================
+
+When the Gecko application is being packaged it bundles a selection of locale resources
+to be available within it. At the moment, for example, most Firefox for Android
+builds come with almost 100 locales packaged into it, while Desktop Firefox comes
+with usually just one packaged locale.
+
+There is currently work being done on enabling more flexibility in how
+the locales are packaged to allow for bundling applications with different
+sets of locales in different areas - dictionaries, hyphenations, product language resources,
+installer language resources, etc.
+
+Multi-Process
+=============
+
+Locale management can operate in a client/server model. This allows a Gecko process
+to manage locales (server mode) or just receive the locale selection from a parent
+process (client mode).
+
+The client mode is currently used by all child processes of Desktop Firefox, and
+may be used by, for example, GeckoView to follow locale selection from a parent
+process.
+
+To check the mode the process is operating in, the :js:`LocaleService::IsServer` method is available.
+
+Mozilla Exceptions
+==================
+
+There's currently only a single exception of the BCP47 used, and that's
+a legacy "ja-JP-mac" locale. The "mac" is a variant and BCP47 requires all variants
+to be 5-8 character long.
+
+Gecko supports the limitation by accepting the 3-letter variants in our APIs and also
+provides a special :js:`GetAppLocalesAsLangTags` method which returns this locale in that form.
+(:js:`GetAppLocalesAsBCP47` will canonicalize it and turn into `"ja-JP-macos"`).
+
+Usage of language negotiation etc. shouldn't rely on this behavior.
+
+Events
+======
+
+:js:`LocaleService` emits two events: :js:`intl:app-locales-changed` and
+:js:`intl:requested-locales-changed` which all code can listen to.
+
+Those events may be broadcasted in response to new language packs being installed, or
+uninstalled, or user selection of languages changing.
+
+In most cases, the code should observe the :js:`intl:app-locales-changed`
+and react to only that event since this is the one indicating a change
+in the currently used language settings that the components should follow.
+
+Testing
+=======
+
+Many components may have logic encoded to react to changes in requested, available
+or resolved locales.
+
+In order to test the component's behavior, it is important to replicate
+the environment in which such change may happen.
+
+Since in most cases it is advised for a component to tie its
+language negotiation to the main application (see `Chained Language Negotiation`),
+it is not enough to add a new locale to trigger the language change.
+
+First, it is necessary to add a new locale to the available ones, then change
+the requested, and only that will result in a new negotiation and language
+change happening.
+
+There are two primary ways to add locale to available ones.
+
+Testing Localization
+--------------------
+
+If the goal is to test that the correct localization ends up in the correct place,
+the developer needs to register a new :js:`FileSource` in :js:`L10nRegistry` and
+provide a mock cached data to be returned by the API.
+
+It may look like this:
+
+.. code-block:: javascript
+
+    let fs = new FileSource(["ko-KR", "ar"], "resource://mock-addon/localization/{locale}");
+
+    fs.cache = {
+      "resource://mock-addon/localization/ko-KR/test.ftl": "key = Value in Korean",
+      "resource://mock-addon/localization/ar/test.ftl": "key = Value in Arabic"
+    };
+
+    L10nRegistry.registerSource(fs);
+
+    let availableLocales = Services.locale.getAvailableLocales();
+
+    assert(availableLocales.includes("ko-KR"));
+    assert(availableLocales.includes("ar"));
+
+    Services.locale.setRequestedLocales(["ko-KR");
+
+    let appLocales = Services.locale.getAppLocalesAsBCP47();
+    assert(appLocales[0], "ko-KR");
+
+From here, a resource :js:`test.ftl` can be added to a `Localization` and for ID :js:`key`
+the correct value from the mocked cache will be returned.
+
+Testing Locale Switching
+------------------------
+
+The second method is much more limited, as it only mocks the locale availability,
+but it is also simpler:
+
+.. code-block:: javascript
+
+    Services.locale.setAvailableLocales(["ko-KR", "ar"]);
+    Services.locale.setRequestedLocales(["ko-KR"]);
+
+    let appLocales = Services.locale.getAppLocalesAsBCP47();
+    assert(appLocales[0], "ko-KR");
+
+In the future, Mozilla plans to add a third way for addons (`bug 1440969`_)
+to allow for either manual or automated testing purposes disconnecting its locales
+from the main application ones.
+
+Testing the outcome
+-------------------
+
+Except of testing for reaction to locale changes, it is advised to avoid writing
+tests that expect a certain locale to be selected, or certain internationalization
+or localization data to be used.
+
+Doing so locks down the test infrastructure to be only usable when launched in
+a single locale environment and requires those tests to be updated whenever the underlying
+data changes.
+
+In the case of testing locale selection it is best to use a fake locale like :js:`x-test`, that
+will not be present at the beginning of the test.
+
+In the case of testing for internationalization data it is best to use :js:`resolvedOptions()`,
+to verify the right data is being used, rather than comparing the output string.
+
+In the case of localization, it is best to test against the correct :js:`data-l10n-id`
+being set or, in edge cases, verify that a given variable is present in the string using
+:js:`String.prototype.includes`.
+
+Deep Dive
+=========
+
+Below is a list of articles with additional
+details on selected subjects:
+
+.. toctree::
+   :maxdepth: 1
+
+   locale_env
+   locale_startup
+
+Feedback
+========
+
+In case of questions, please consult Intl module peers.
+
+
+.. _RFC 5656: https://tools.ietf.org/html/rfc5656
+.. _BCP 47: https://tools.ietf.org/html/bcp47#section-2.1
+.. _ISO 639: http://www.loc.gov/standards/iso639-2/php/code_list.php
+.. _ISO 3166-1: https://www.iso.org/iso-3166-country-codes.html
+.. _Intl.Locale: https://bugzilla.mozilla.org/show_bug.cgi?id=1433303
+.. _fluent-locale: https://docs.rs/fluent-locale/
+.. _bug 1440969: https://bugzilla.mozilla.org/show_bug.cgi?id=1440969
new file mode 100644
--- /dev/null
+++ b/intl/docs/locale_env.rst
@@ -0,0 +1,34 @@
+Environments
+============
+
+While all the concepts described above apply to all programming languages and frameworks
+used by Mozilla, there are differences in completeness of the implementation.
+
+Below is the current list of APIs supported in each environment and examples of how to
+use them:
+
+C++
+---
+
+In C++ the core API for Locale is :js:`mozilla::intl::Locale` and the service for locale
+management is :js:`mozilla::intl::LocaleService`.
+
+For any OSPreference operations there's :js:`mozilla::intl::OSPreferences`.
+
+
+JavaScript
+----------
+
+In JavaScript users can use :js:`mozilla.org/intl/mozILocaleService` XPCOM API to access
+the LocaleService and :js:`mozilla.org/intl/mozIOSPreferences` for OS preferences.
+
+The LocaleService API is exposed as :js:`Services.locale` object.
+
+There's currently no API available for operations on language tags and Locale objects,
+but `Intl.Locale`_ API is in the works.
+
+Rust
+----
+
+For Rust Mozilla provides a crate `fluent-locale`_ which implements the concepts described
+above.
new file mode 100644
--- /dev/null
+++ b/intl/docs/locale_startup.rst
@@ -0,0 +1,97 @@
+Startup
+=======
+
+There are cases where it may be important to understand how Gecko locale management
+acts during the startup.
+
+Below is the description of the `server` mode, since the `client` mode is starting
+with no data and doesn't perform any operations waiting for the parent to fill
+basic locale lists (`requested` and `appLocales`) and then maintain them in a
+unidirectional way.
+
+Data Types
+----------
+
+There are two primary data types involved in negotiating locales during startup:
+`requested` and `available`.
+Throughout the startup different sources for this lists become available, and
+in result the values for those lists changes.
+
+Data Sources
+------------
+
+There are three primary sources that become available during the bootstrap.
+
+1) Packaged locale lists stored in files :js:`update.locale` and :js:`multilocale.txt`.
+
+2) User preferences read from the profile.
+
+3) Language packs installed in user profile or system wide.
+
+Bootstrap
+---------
+
+1) Packaged Data
+^^^^^^^^^^^^^^^^
+
+In the `server` mode Gecko starts with no knowledge of available locales, nor of
+`requested`.
+
+Initially, all fields are resolved lazily, so no data for available, requested,
+default or resolved locales is retrieved.
+
+If any code queries any of the APIs, it triggers the initial data fetching
+and language negotiation.
+
+The initial request comes from the XPCLocale which is initializing
+the first JS context and needs to know what locale the JS context should use as
+the default.
+
+At that moment :js:`LocaleService` fetches the list of available locales using
+packaged locales which are retrieved via :js:`multilocale.txt` file in the toolkit's
+package.
+This gives LocaleService information about which locales are initially available.
+
+Notice that this happens before any of the language packs gets registered, so
+at that point Gecko only knows about packaged locales.
+
+For requested locales, the initial request comes before user profile preferences
+are being read, so the data is being fetched using packaged preferences.
+
+In case of Desktop Firefox the :js:`intl.locale.requested` pref will be not set,
+which means Gecko will use the default locale which is retrieved from
+:js:`update.locale` file (also packaged).
+
+This means that the initial result of language negotiation is between packaged
+locales as available and the default requested locale.
+
+2) Profile Prefs Read
+^^^^^^^^^^^^^^^^^^^^^
+
+Next, the profile is being read and if the user set any requested locales,
+LocaleService updates its list of requested locales and broadcasts
+:js:`intl:requested-locales-changed` event.
+
+This may lead to language renegotiation if the requested locale is one of the packaged
+ones. In that case, :js:`intl:app-locales-changed` will be broadcasted.
+
+3) Language Packs Registered
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Finally, the AddonManager registers all the language packs and they get added to
+:js:`L10nRegistry` and in result update LocaleService's available locales.
+
+That triggers language negotiation and if the language from the language pack
+is used in the requested list, final list of locales is being set.
+
+All of that happens before any UI is being built, but there's no guarantee of this
+order being preserved, so it is important to understand that depending on where the
+code is used during the startup it may receive different list of locales.
+
+In order to maintain the correct locale settings it is important to set an observer
+on :js:`intl:app-locales-changed` and update the code when the locale list changes.
+
+That ensures the code always uses the best possible locale selection during startup,
+but also during runtime in case user changes their requested locale list, or
+language packs are updated/removed on the fly.
+
--- a/intl/moz.build
+++ b/intl/moz.build
@@ -45,8 +45,10 @@ with Files("icu-patches/**"):
 with Files("tzdata/**"):
     BUG_COMPONENT = ("Core", "JavaScript: Internationalization API")
 
 with Files("update*"):
     BUG_COMPONENT = ("Core", "JavaScript: Internationalization API")
 
 with Files("icu_sources_data.py"):
     BUG_COMPONENT = ("Firefox Build System", "General")
+
+SPHINX_TREES['/intl'] = 'docs'