I've read a few million words of French—nothing special, but enough that if a text gives me trouble, then it's also likely to break Google Translate. Over the years, I've collected a few example texts which give Google Translate an especially hard time, and I use those for testing other machine learning systems.

DeepL performs significantly better on the most difficult texts I've given it. It's substantially better with colloquial language, and—oddly—nautical language. It also seems to be much better at tracking relationships between words in longer sentences. Good work!