So I finally found an app that works like a charm for me. Lernbox App
It is capable of opening txt files and check automatically if there are new entrees copying them to the vocabulary. So I always open the app with a shortcut which loads the txt file, so its always up to date. For adding new vocabulary, I use the shortcuts above, so every word I look up is automatically added to the txt-file. That works pretty great!
Edit: So for completion, here the whole setup. You need a file, Vokabeln.txt in the shortcuts directory and the app Lernbox:
Transalting to German
Translating to English
Vocabulary Training
Menu to have all at your fingertip