The mozillathunderbird.desktop file only contains the English entry. Reproducible: Always Steps to Reproduce: Expected Results: Translations in the .desktop file.
Created attachment 34987 [details, diff] Add Hungarian translation and use UTF-8 encoding.
please note that this should also be added to the mozilla-thunderbird-bin package (all .desktop file must have an 'Encoding=" line, quite always "Encoding=UTF-8")
Don't really see why we should be including Hungarian translation... Why not Czech or Japanese or another 200 languages?