uninstall doesn't remove language (.mo) files

[andy5995]

Right now, there's a script (uninstall_langs.sh) that removes the .mo files that are installed when ninja install is run.

Fortunately, uninstalling the mo files has just been added to meson (mesonbuild/meson#1601 (comment)).

I think we'll need to wait a while before removing the uninstall_langs.sh script, until some distributions catch up with the latest version of meson.