Rework translation system to use PO files

The French translations are provided by Tanavit. Unfortunately I was not
able to keep this addition as a separate commit as the translation file
was originally added as a .tr file that I then converted to .po file in
the meantime.

Also note that this commit is created from squashing 20+ commits from
the l10n branch that preceded the transition to PO files. In addition to
changes to the locale files (which were all included in the single
commit for transitioning to PO files), these commits also included code
that has now become obsolete for l10n work. In particular, it included a
GUI program written in Tcl to edit .tr files; this program can now be
found in the following repo: https://codeberg.org/y5nw/mt_tr_editor

Co-authored-by: Tanavit <tanavit@posto.ovh>

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