From MozillaZine Knowledge Base
Sometimes if you run Thunderbird the screen displays the following near the bottom of the main window:
Its a reference to text that is stored in a separate file in order to make it easier to translate. When Thunderbird tried to look up the text in your current language it could not find it, so it complained.
This problem normally occurs if you upgraded Thunderbird in the same Thunderbird program directory and for some reason some of the prior versions files were not deleted beforehand. Uninstall Thunderbird, check that the Thunderbird program directory is empty and install it again.
It might also occur if you installed a buggy extension or theme. Try running Thunderbird in safe mode to temporarily disable any added extensions/themes and see if the problem goes away. If so, uninstall the bad extension or theme.