Avoid this construction if possible. The first one is acceptable, although incorrect. The second is wrong.
Technically we should say, I'm late, am I not?
In the case of "aren't," the standard pronunciation is to not pronounce the 't.' So when you say "I'm late, aren't I?" it is typically pronounced as "I'm late, a-n-r-I." However, there may be regional variations in pronunciation, so keep in mind that different people may pronounce it differently. As for "ain't," it is considered a colloquial and informal contraction of "am not," "is not," or "are not." Strictly speaking, "ain't" is not considered as grammatically correct in formal writing or standard English usage. So, in summary, for the phrase "I'm late, aren't I?" you generally do not pronounce the 't,' but please note that pronunciation may vary in different contexts and regions.
