elixir gettext current does not fully support fallback to another language [0].
But it might in the future. We adapt it so that all languages in Accept-Language
headers are received by Pleroma.Web.Gettext. User.languages is now a comma-separated
list.
[0]: https://github.com/elixir-gettext/gettext/issues/303
For an example, here, zh is not supported, but zh_Hans and zh_Hant
are. If the user asks for zh, we should choose a variant for them
instead of fallbacking to default.
Some browsers (e.g. Firefox) does not allow users to customize
their language codes. For example, there is no zh-Hans, but only
zh, zh-CN, zh-TW, zh-HK, etc. This provides a workaround for
those users suffering from bad design decisions.
