Fix unstable GNU translations

Robin Candaurequested to merge
fix_unstable_translations into main

GNU does not include PO files (used for translations) in their git repositories. When building GNU software from git sources, PO files are sourced from https://translationproject.org/ in an unstable way (even pointing to a specific tag / release doesn't guarantee stability, see #3).

This is problematic because:

  • It implies file downloading during the build process (which should generally be avoided)
  • It may break reproducibility
  • It may ship unstable / unmatching translation files to users

Fully switching back to dist tarball as the primary source for the package is not desirable as per our guidelines established in RFC 46. As such, we are only building & installing translation files from the dist tarball, the software itself is still compiled against git / "transparent" sources.

See the related ToDo list (and additional links there) for more details and rationale.

Fixes #3

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