Confirming on locale fr_FR.UTF-8. I just did the migration 10.04->10.10->11.04->11.10->12.04.
The problem is that the thunderbird-locale-xx is not automatically installed during system upgrade.
Even using the gnome-language-selector does not fixes the problem.
Confirming on locale fr_FR.UTF-8. I just did the migration 10.04-> 10.10-> 11.04-> 11.10-> 12.04.
The problem is that the thunderbird- locale- xx is not automatically installed during system upgrade. selector does not fixes the problem.
Even using the gnome-language-
Workaround: manually install the missing package.