I18N: Support more valid locales: language-script and language-script-region

[hanguokai]

Locale Format

Locale identifier consists of language subtag, script subtag, region subtag, and one or more variant subtags.

For example,

• zh-CN and zh-TW are language + region. They mean Chinese in different regions.
• zh-Hans and zh-Hant are language + script. They mean Simplified Chinese and Traditional Chinese without regions.
• zh-Hans-CN, zh-Hans-SG, zh-Hant-HK and zh-Hant-TW are language + script + region. They mean Simplified Chinese and Traditional Chinese in different regions.

For historical reasons, zh-CN and zh-TW are often used to represent Simplified Chinese and Traditional Chinese, but this is not rigorous or accurate.

The current level of support

• Chrome: support a list of locales, i.e. language + region only.
  1. It does not support script and script+region.
  2. It doesn't support all languages (e.g. some minority languages) and all regions (e.g. Hongkong).
• Firefox: similar to Chrome.
• Safari: after a recent change, support script and script + region.

Goal

All browsers support all valid locales, including:

• language + region
• language + script
• language + script + region

Note: The languages supported by the browser and the languages supported by the extension store should be two different things, but there is some correlation.

Other discussions

• Proposal: Per-extension language preferences #641 We also talked about support for locales, and what locales the setCurrentLanguage() allows to be passed in.
• i18n.getMessage() language fallback paths #296 @birtles and @xeenon discussed this issue. And related links: zh_hans locale not working birchill/10ten-ja-reader#2051 https://issues.chromium.org/issues/375528194
• Support for Hong Kong in the i18n settings

[carlosjeurissen]

Also relevant: #131
Which compiles a list of the current situation in terms of what locale tags are supported by what browser/stores:
https://docs.google.com/spreadsheets/d/1N0f9Nl-PM_nafpJEfYbS8xV9Qvb6o6DvuBFWLTXRr2Y/edit#gid=1518318975

We should also align on properly formatted locale tags:
#642

The webExtension Stores should also receive support for this. Currently the Naver Whale extension store does not support loading extensions with locale tags which include script subtags (like sr-Latn or zh-Hans).

[carlosjeurissen]

@hanguokai in terms of support. What do you expect from browsers? Say the browser uses "zh-CN" as locale. And the extension only has "zh-Hans" and "zh-Hant", should it actively search for a best match and pick "zh-Hans"? Or should it just be able to accept extensions which have locale files using language tags containing a script subtag?