Hello Carsten. Thanks for your response. Here is some feedback. Locale: LANG=en_CA.utf8, LC_CTYPE=en_CA.utf8 (charmap=UTF-8), LANGUAGE=en_CA:en Your local language ist set to en_CA, not en_GB. Thunderbird is using this locale setting from the system. No, Thunderbird is not using en_CA. If it was there would be no problem. The user interface of Thunderbird, despite the package thunderbird-l10n-en-gb being installed and enacted, is most definitely en_US. I should note that I do not even have the en_US locale available on my machine: mark@debian:~$ locale -a C C.UTF-8 en_CA en_CA.iso88591 en_CA.utf8 en_GB en_GB.iso88591 en_GB.iso885915 en_GB.utf8 POSIX
You need to run Thunderbird with a different setting of the LANG variable if you want to use a different UI language than your local locale setting. This can be simply done by adjusting the LANG environment to the desired language, for example on the cli: $ LANG=en_GB.utf8 thunderbird I don't know the specific diffences between en_CA, en_US and en_GB so I can't say what the Ui needs to show in which menu. As far as spelling, there are greater similarities between en_CA and en_GB than there are between en_CA and en_US (true for many English speaking commonwealth countries). Generally, from what I have seen, large programs that do not have language user interfaces that respond to the locale will provide their own language user packs. Libreoffice does this, and that language pack (libreoffice-l10n-en-gb) works flawlessly.
