I have the laptops with ubuntu mate 18.04 and thunderbird (from the repo) installed. All of them have a very old thunderbird profile. All of them had a old unsupported lightning addon installed. I removed it and restarted thunderbird. The "new" Lightning appeared. But: Thunderbird is in german, but lightning is in english. What i have done: 1. the package from the repo (thunderbird-locale-de) is installed and ther german language package is shown under addons. 2. about:config 2.1 intl.accept_languages -> de, en-US, en (I tried only de and de-DE) 2.2 I added a new string: general.useragent.locale -> de Nothing works. Ho can i change lightning to german?