There is an algorithm determining the language of the SIGNL4 mobile app. The phone's operating system manages a list of user-installed languages (it is similar on iPhone and Android).
SIGNL4 currently supports English and German. By default, the app is displayed in English.
If German is found anywhere in this list of supported languages, the app is displayed in German. Except for the case that German is listed after English, in this case the app is displayed in English too.
So, SIGNL4 always picks the topmost of installed languages.