I used it for practice Spanish. LLMs have no issues with generating decent sentences in it. Also as I practiced vocabulary building, I didn't need specific grammar constructs or even to be very sensitive to the occasional mistakes in this area (which I don't remember to notice).
It took me quite a bit of iterations to get AI to generate output I found good enough and with acceptable errors rate for my practice but that was the part of fun.
If anybody wants to try the same route, the biggest mistake not to repeat is to trust LLMs to generate words definitions by them itself. It does it quite well, but still gives wrong definitions from time to time (~1% in my case for the OpenAI models 2 years ago). This wasn't acceptable to me and took a lot of effort to review and correct cards I already had. Feed vocabulary article with word definition to LLM instead if you want it to generate word definitions suitable to your level instead.