Have you had the opportunity to interact with less wrapped versions of the models?
There's a lot of intentionality behind the way LLM's are presented from places like ChatGPT/DeepSeek/Claude, you're distinctly trying to talk to something that's actively limited in the way it can speak to you
It's not exactly nonexistant outside of them, but they make it worse than it is
Does it matter? Even most Chinese models are trained with <50% Chinese dataset last I checked, and they still manage to show AliExpress accent that would be natural for a Chinese speaker with ESL training. They're multilingual but not agnostic, they just can grow English-to-$LANG translation ability so long English stays the dominant and defining language in it.
I've run a bunch locally, sometimes with my own takes on system prompts and other adjustments because I've tried to make them less insufferable to use. Not as absurdly submissive, not as eager to do things I've not asked, not as censored, stuff like that.
I find they struggle a lot with things like long sentences and advanced language constructs regardless of the natural language they try to simulate. When it doesn't matter it's useful anyway, I can get a rough idea about the contents of documents in languages I'm not fluent in or make the bulk of a data set queryable in another language, but it's like a janky hack, not something I'd put in front of people paying my invoices.
Maybe there's a trick I ought to learn, I don't know.
It's not exactly nonexistant outside of them, but they make it worse than it is