Merge branch 'toolforge_i18n'

This moves the translation framework into a separate library. It’s
currently included as a Git submodule because it was easiest this way
(first develop it in a subdirectory, then turn that whole directory into
amodule); I’ll eventually turn it into a real package that other tools
can easily use too.

Some improvements to the framework are also included, e.g. ?uselang=qqx
support.

Bug: T363626