Setting site language to English
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You can see that the UI language is still German. But more important: Even the labels of the buttons inside the exercise are in German ("Quiz starten" instead of "Start quiz").
Solution
See my comment at the H5P Forum.
change this function to always return 'en', i.e. just remove the 10 other lines in the function you should always get English.
https://h5p.org/comment/32586#comment-32586