Updates mess with thunderbird translations (yes/no buttons)

Hi, in thunderbird, when one uses the keyboard shortcut "n" to navigate to the next unread message and the next unread message is in another folder a window opens asking for confirmation to actually go to the next folder. Now my system is entirely configured for the German language settings and should therefor label the buttons with Ja/Nein. Now I realized a few times that when I run updates, they mess with these buttons, they then appear in English language as Yes/No while the rest of thunderbird (as far as I can tell) still uses German language. Depending on the update I have different language on these buttons... Two questions: - How can I set that back? - Is there a way to prevent this? thanks, Karl