Idk. First, the obvious candidates: those with small, low quality text corpora. Second, I expect LLMs to perform less on long distance dependencies. English has a pretty strict word order, and the constituents are close to each other. There's also a lot of input available, so there are more large gaps to learn from. But (some) Germanic languages have some weird rules which allows constituents to move far away, so it's likely LLMs will make more errors there. Also, free word order languages might be a problem, although I've always suspected that those languages aren't that free in reality: speakers probably have strong preferences.

Any with inflections and particularly with grammatical gender.

If I had to guess, from my time working with ASR (Advanced Speech Recognition) languages and its difficulty with the Indian (country of) accent; I'd guess Indian languages.