Someone who uses really bad vocabulary and grammar but not because they are esl but for examplein black communities I’ve heard people say things like “they is bad” or in Mexico they say “ira” instead of “mira(look)” or “chescas” instead of “sodas”, ira and chescas are low class word and we call that people “nacos”, how would you call someone like that
We don't really have a general derogatory term for such speakers, perhaps because most of us don't regard them with particular contempt.
In English most such usages are not "really bad vocabulary and grammar". These speakers are not trying to use Standard English (whatever that is) and failing. They are using ordinary colloquial English or, as in your example, forms which are standard in their dialects.
"Really bad vocabulary and grammar" is what you encounter when people try to imitate a dialect without understanding its rules. (This doesn't only mean failing to speak or write academic English or bureaucratic English or literary English correctly; you can just as readily embarrass yourself and your hearers by making a hash of African-American Vernacular or teenage slang.) We don't call such people bad names; we just wince and carry on.
There are many way call a person a "naco" but generally it means to be bad mannered or poorly educated people that use "slang" meaning that they are words that consists of words and phrases and may be regarded as very informal