Add ability to change Firefox Preview (browser UI) language

[bbinto]

Why/User Benefit/User Problem

As a user, I want to experience the app in a language other than the system / phone language

Deps

• AC API for switching languages

Acceptance Criteria (how do I know when I’m done?)

• I have the ability to change browser UI language separate from the system language
• Browser language will follow the system language by default

Mocks: https://miro.com/app/board/o9J_kwN9u-I=/
SVG asset for Language icon: https://dzwonsemrish7.cloudfront.net/items/1j0I1L1E282t24241N0l/language.svg

NOT in this issue

• Add ability to select Webpage Content Language #6502 Allow webpage content language selection
• expanding the locales we currently support Enable new locales in nightly #5496

This is just scoped to making the picker inside of Fenix, NOT inside AC

┆Issue is synchronized with this Jira Task

[cpeterso]

Issue #940 confirmed that Fenix's app UI language changes correctly when the user changes the Android system language.

AFAICT, Chrome's Language settings only configure the user's preferred language for web content (i.e. the Accept-Language HTTP header sent to web servers), not Chrome's app UI language.

GV bug https://bugzilla.mozilla.org/show_bug.cgi?id=1543823 is about letting the browser customize Gecko's Accept-Language HTTP header and/or making Gecko use the Android system language.

UX question to consider:

1. Assume Android system language is en-US.
2. User changes Fenix's language setting to en-CA.
3. User then changes the Android system language to en-GB.
4. Should Fenix's language setting preserve its non-default setting (en-CA) or switch to new system language (en-GB)?

[yoasif]

1. Assume Android system language is en-US.
2. User changes Fenix's language setting to en-CA.
3. User then changes the Android system language to en-GB.
4. Should Fenix's language setting preserve its non-default setting (en-CA) or switch to new system language (en-GB)?

It feels to me like Fenix should be en-GB and and websites should be en-CA unless Fenix implements separate website and browser UI preferences for language.

Also, FWIW, Chrome also allows users to set up several languages based on priority, while Fennec only allows users to select a single one (likely because the preference applies to both UI and websites).

[liuche]

@boek will we have to wait for the new AC nightly version to be merged into Fenix? Not sure how to track, I'll add a comment to #9413

[boek]

@liuche I updated a-c with this PR :)

[lobontiumira]

Verified that the user can:

• change the language in Fenix to be different than the system's language,
• follow the system's language,
• change the language in Fenix with dark or light theme applied.

Tested with Google Pixel (Android 10), OnePlus 5T (Android 9), Sony Xperia Z5 Premium (Android 7.1.1), Samsung Galaxy Tab A6 (Android 5.1.1), and HTC 10 (Android 8) on the latest Nightly build from 4/8.

I'll remove the qa:needed label.

@liuche #9413 is not entirely fixed, should I leave this issue opened as well?

[liuche]

Took a look at that other bug - thanks for filing and following up! For adding the language-switcher, I think we can fix that in release - it affects not all devices, but RTL support is important to get right. So I'll close this, and we can follow up in the other bug.