Prompting a good LLM to convincingly act like a native isn't hard, neither is jailbreaking it if necessary. The hard thing in this case is verifying that it really does that.
You're assuming prompting an LLM to behave like X will automatically result in the LLM behaving like X for any X. Some things are out of the LLMs ability: I'm sure you'll get something if your prompt is "You are a 300-IQ nuclear physicist with a doctorate in material science. Describe the design of a cold-fusion reactor", but there is no guarantee the LLM has fidelity to your prompted persona, though it will try to give its very best impression.
I'm not assuming, I'm a native speaker and I describe actual experience with Claude. It's pretty good at roleplaying and major languages, with certain caveats. If you aren't a native speaker though, you'll have a hard time verifying that it gives you a good result, that's what I'm trying to say.
Nah, it's not reproducible. It can certainly give you some common phrases, but to create an entire personality with its own set of speech patterns is a different thing. For example, Claude can suggest following friendly conversation starters to sound like a native (a quote below):
Popular combinations for texts:
"Прив! Чё как?" - "Hi! How's it going?"
"Дарова, живой?" - "Hey, you alive?"
"Салют! Как сам?" - "Hey! How are you?"
Modern slang (especially among younger people):
"Хай" (Khay) - borrowed from English "hi"
"Йоу" (You) - borrowed from English "yo"
When asked, which one it would pick, it goes with "Дарова, как сам?" (Darova, kak sam?) and that already sounds odd in many contexts.
Obviously not. But this is what LLM may give you, unless you know exactly what to ask for (which you will not know without the proper knowledge of language and cultural context).