Basically, the FocusTimer set its language according the device language if you’re using English, Korean, Chinese, Japanese. (As of v1.6.x).
If you want to set it by manually, you can change it on the setting view.
There is a language cell among general section. ‘Laguage’ is always presented in English no matter what the current language is set.
You can set the languages among English, Korean, Chinese and Japanese. (As of v1.6.x, Late 2017)