Default UI language is resolved in the frontend

[veloman-yunkan]

Fixes #1038 Fixes #1039

This change makes the viewer resolve the default UI language in the front end, without any dependence on the backend's handling of the Accept-Language header. Yet, the latter has not been removed since it is currently used in the following scenarios:

• /nojs endpoint
• error responses (e.g. /invalid_endpoint, or any other invalid kiwix-serve URL directly loaded in the browser rather than via the Kiwix JS viewer).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (aff801e) 39.34% compared to head (c697611) 39.35%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1044 +/- ## ======================================= Coverage 39.34% 39.35% ======================================= Files 58 58 Lines 4061 4060 -1 Branches 2237 2236 -1 ======================================= Hits 1598 1598 Misses 1091 1091 + Partials 1372 1371 -1 ```

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[veloman-yunkan]

Reminder to myself - translatewiki PR #1033 has been merged, so languages.js must be regenerated during rebase&fixup.

[mgautierfr]

Please rebase-fixup.

[veloman-yunkan]

Reminder to myself - translatewiki PR #1033 has been merged, so languages.js must be regenerated during rebase&fixup.

Done

[veloman-yunkan]

Please rebase-fixup.

@mgautierfr Done