User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Firefox/45.0 Build ID: 2016030500 Steps to reproduce: My thunderbird and lightning extension get updated sometime. After that, Lightning can only show English user interface, but other parts in Thunderbird is in Simple Chinese. Chinese translation is in finished status in Mozilla i18n dashboard. Cannot find a way to switch Lightning to Simple Chinese.
Are you using an English version of TB with an additional language pack from AMO? In that case this is an expected side effect throughout the TB 38 cycle (and maybe also 45). The bundled Lightning is single language packaged only and depends on the language of your TB installation, while the Lightning version available on AMO is multi language packaged. In the TB 38 (and maybe also in 45) cycle we had to do additional AMO releases. Switching between bundeled and AMO version may have triggered that toggling if you have an English version of TB installed and use the TB language pack from AMO on top of this. Install a Chinese version of TB instead to avoid that behaviour.
I am using Simple Chinese version of Thunderbird. The Lightning is bundled in Thunderbird, from openSUSE Linux package manager. I tried to uninstall Lightning and reinstall it from AMO. Now it display Chinese! I think it is the problem of the bundled Thunderbird. When Linux package manager updated Thunderbird and Lightning is newer than what I installed, the Lightning add-on will be replaced by the one bundled in Thunderbird and become English.
Maybe the best solution is to bundle full language Lightning to Thunderbird. So this bug is still valid.