Client-side translations: gettext and plural support

[Ekdohibs]

This PR is a rebase of #10451.

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Add gettext support for client-side translations. This allows using existing gettext tooling, and will allow using plurals in translated strings (ngettext and the like).

Testing is needed, but the PR is otherwise complete.

[Ekdohibs]

I added a commit for testing, and fixed a few bugs that were still there that I found.
To test:

• add the following to po/fr/minetest.po (this is not committed, as weblate takes care of translations):

msgid "0"
msgid_plural "1"
msgstr[0] "0"
msgstr[1] "1"

• set your locale to "fr"
• start the devtest game and run /testtranslations.

This should test all three types of translation files, as well as plural translations for both .po and .mo files (plurals are unsupported in .tr files).

[Zughy]

@Ekdohibs please rebase
core devs: please show interest

[Wuzzy2]

Was this closed due to success or due to failure?

[Zughy]

@Wuzzy2 author has disappeared. Also, more than 30 days after having asked for a rebase

[Wuzzy2]

Please add the "adoption needed" label, just in case.