I've used gpt4 pretty extensively while learning Japanese to have a written conversation partner, ask clarification around grammar, or to translate native content for me and explain it. I've validated a lot of the answers it comes up with, and although it hallucinates occasionally, it doesn't do so on the same points consistently. I'm going to encounter a lot of my vocabulary and grammar hundreds or thousands of times, so even if it's incorrect 1% of the time, it's not a huge problem.

As with most LLM use cases, it's best when used to augment an existing workflow that reinforces it. In my case, I already have an whole setup where I'm using anki flash cards for vocabulary and grammar study, some curated human-written resources for learning grammar, and native language content for reading and listening immersion. GPT is really helpful to be able to quickly get a sentence level translation and each word translated, and full descriptions of the grammar points at work in the sentence. It saves me a lot of time over working with a dictionary and juggling grammar resources, vocab, etc. I can ask it follow-up questions, and even switch straight into trying to use the grammar/vocab in an example sentence of my own right on the spot. I seriously think I'd be way worse off if I didn't have access to an LLM throughout the process.