It depends on who you ask, but according to syntacticians, specialists in syntax or sentence structure, as long as a sentence follows a recognisable structure and is used by people it is grammatically correct. It's grammatically correct within that dialect.
Most people would disagree and say that dialects are ungrammatical because they deviate from standard English, but this is probably because they want you to learn to speak English to a certain standard. From an official standpoint, dialects are just as grammatical as standard English.
