fix: Localisation-matching algorithm missing some edgecase

[CommanderStorm]

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Description

Resolves #4691
Resolves #4690
Resolves #4697

There were these minor bugs in the new language selection mechanism introduced in 1.23.12:

• we only fall back to the generic locale (for en-US this would be en) for the first entry of the languages
• We never match en if other (less prefered) alternatives exist in the users preferences, as this is only in the messages and not in the languageList.
• We assumed that the generic locales are only 2 characters, not 3

Our testcases missed this edgecase

thanks @CirnoT for the bug report

Type of change

Please delete any options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas
  (including JSDoc for methods)
• My changes generate no new warnings
• My code needed automated testing. I have added them (this is optional task)

[Saibamen]

This is the same test case as in line 35