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+.. 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