(Adoption) Gettext and (partial) plural support for client-side translations

[y5nw]

This is an adoption of #13687.

• Goal of the PR
  Implement support for .po and .mo files for client-side translations.
• How does the PR work?
  This PR includes parsers for po and mo files, which can be used regardless of whether Gettext is used or not.
• Does it resolve any reported issue?
  Fixes Switch to Gettext (PO) file format for client-side translations #7290
• Does this relate to a goal in the roadmap?
  I do not think so.
• If not a bug fix, why is this PR needed? What usecases does it solve?
  See linked issue.

To do

This PR is a Work in Progress.

• Support plural forms based on the Plural-Forms field of the header entry.
• Support fuzzy entries: Client-side translations: gettext and plural support #13687 (comment)
• Address missing translation in content page: Client-side translations: gettext and plural support #13687 (comment)
• Add tests for:
  • Parsing entries without msgctxt
  • Skipping fuzzy entries
  • minetest.get_translated_string
• Add documentation

How to test

Use /testtranslation with Minetest in a French locale.

[rubenwardy]

Thanks for adopting this!