Hi. I'm really curious to find out about natives' impression of a foreigner who is trying to imitate their accent and culture and surely making some errors.
In your opinion, does it seem awkward?
Do you prefer to see them talking in a simple and polite way with the least mistakes rather than a complex way with quite a lot of wrong words and structures?
How natural does it look to see them using some slang, informal idioms and vulgar phrases?
Good question! I know an American who speaks German with a Bavarian accent. It definitely surprised me, but I didn't find it awkward. Some people just pick up accents, there's nothing wrong with that. Accent has a lot to do with pronunciation. I'd think it's better to pronounce a word like a native speaker, this way you're certain to pronounce the word in a correct manner and avoid confusion with other, similar sounding words. Of course there different types of accents. The student I mentioned had a slight Bavarian accent. But there are accent variations which can be incomprehensible to people not from the same area even with the same native language. I wouldn't advice learning to do that without also being able of speaking an "understandable" variation unless you plan to stick to the one area and its population where that accent is spoken.
As for simple vs complex sentences: most people's goal is (or should be) to be understood. I think it's better to use simple and polite language, where you make fewer mistakes and are more likely to be understood. Of course, if your goal, during a conversation, is to learn, then you should definitely move out of your comfort zone and use more complex words and structures to learn them better.
I think it's also perfectly fine for learners to use slang and swear words but they need to understand them and use them with care. I believe that people should have a good grasp of the standard language before using slang or swear words. This is also to avoid offending people: swear words in a foreign language don't have the same emotional impact on you as swear words in your native language; so you might not be able to judge their impact very well and are more likely to offend people.