fix(l10n): fall back to base language

[susnux]

• resolves Public File Request guest identification dialog is not translated into Japanese #2581

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• The content of this PR was partly or fully generated using AI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.21%. Comparing base (0f0b489) to head (1692068).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2619      +/-   ##
==========================================
+ Coverage   61.74%   62.21%   +0.47%     
==========================================
  Files          15       15              
  Lines         481      487       +6     
  Branches      104      107       +3     
==========================================
+ Hits          297      303       +6     
  Misses        165      165              
  Partials       19       19              

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[Antreesy]

Shouldn't the split be the other way around? e.g. we have de.js and de_DE.js, but not de_AT.js; then:

• useBaseLanguage is true
• needsRename is false, because currentLanguage is de_AT and both language.split('_')[0] are de
• better match should be more generic, so de and not de_DE?

I see how it works with current ja and available ja_JP though

[susnux]

This is more a future proof variant of hard renaming done in server:

lang_map = fi_FI: fi, hu_HU: hu, nb_NO: nb, sk_SK: sk, th_TH: th, ja_JP: ja, bg_BG: bg, cs_CZ: cs

We can also just hardcode that map if that makes more sense

[susnux]

See: #2620

[Antreesy]

Ok, didn't know it's from .tx/config