Andrés, indeed upstream language packs are built differently than the Ubuntu locale packages for thunderbird, they don't have the same content, and that's what is causing the problem.
What I haven't managed to understand yet is how/why thunderbird downloaded language packs on upgrade, overriding the ones that are installed system-wide.
Andrés, indeed upstream language packs are built differently than the Ubuntu locale packages for thunderbird, they don't have the same content, and that's what is causing the problem.
What I haven't managed to understand yet is how/why thunderbird downloaded language packs on upgrade, overriding the ones that are installed system-wide.