Because LT (https://languagetool.org) was mentioned in the Keyboard Maestro Discourse (https://forum.keyboardmaestro.com/t/languagetool-3-9-not-available/8330), I had a look at it. First I noticed that I couldn't test it with Java 9. I had to use Java 8 (this seems to be known in the LT forum).
Anyway, I was actually quite disappointed by the "linguistic suggestions" that it offered for Dutch.
Does it provide a better checking for other languages?
Spanish is impressively good.
German is quite okay.