Advanced preferences, language dropdown in basic preferences

[AntonKhorev]

The goal was to get #5201 to a mergeable state, but its language dropdown is in suboptimal state. So I decided to take a second attempt at #4461 which also has a language selector. If the attempt is successful, the selector can be reused in #5201.

The problems in #5201 with the language selector were:

• Forcing translators to translate every language name into every language;
• Throwing away some languages because of case-sensitive matching;
• Showing languages in the current locale which you may not be able to read. This should be a problem for a computer in India scenario from Language switcher on the homepage #1115, where they have not just different languages but different scripts too. Anyone selects a wrong script, and then nobody is able to read anything, language names included.

The list in this PR is generated using this idea #5201 (comment), by reading native language names from comments in yml files. If we're not sure whether these comments are going to stay (the files are updated by TranslateWiki), we can dump them to some other file and read from there (UPD: done). Note that the UI language set differs from the diary language set in config/languages.yml. The resulting language dropdown shows native language names in case/diactric-insensitive order.

Fixes #2581 by showing all available UI languages in a dropdown. You might still want to specify other languages for displaying name tags, that's what the language input on the Advanced Preferences page is for. I tried putting everyting on one page, but that complicates the user interface as you can see in #4461. A one-page solution would require either showing two language inputs at the same time or adding some button to switch between them, requiring javascript. The solution here doesn't require extra javascript.

[github-actions]

	1 Warning
⚠️	Number of updated lines of code is too large to be in one PR. Perhaps it should be separated into two or more?

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[tomhughes]

I guess a static file works as well at least for now, so I think this is good to merge, thaks.