This is great. How did you do the frequency groupings for more complex languages like German or Icelandic? Did you use a word stemmer or did you find lists that include noun declensions?
Do you also detect word types? I.e. "Der Mann" (nominative masculine article der) vs. "Das Haus der Frau" (genitive feminine article der) [German] or "að leita að" (infinitive marker að vs. prepositional að) [Icelandic]
At the moment Pro users can play sentences grouped by missing word part-of-speech for German, Spanish, French, and Italian from English. I've tried a few different approaches to determining the word types / part-of-speech of words in a given sentence, but none have been super accurate so far. There's definitely a lot more fun/interesting stuff I'd like to do with Clozemaster if I ever do find or develop a good way to detect types.