Whether a person from Turkey understands 'ain't', I don't know. If they have a high level of English, they will. If not, then they won't. But forget that for a minute. This phrase sounds very rude in the context you describe. If you said this to a native speaker in this context they would think you are a moron. Simple as that. It is dismissive. As though you think the project is a waste of time. Forget the non-standard grammar (which I know you already know, and I know you have been advised against using it), this phrase is completely inappropriate in this context. By the way, did you see the discussion about 'ain't' the other day? I know you much prefer American English. The Americans who commented on that discussion were really quite vociferous in their, (I know it's a strong word but in this case I think it fits) hatred of its usage.

Don't use it, either with natives or non-natives. Then you don't need to wonder about it. "Ain't" is very complicated. It's not a "bad word," but when a native speaker hears it, we must first decide, "is this person uneducated, or is he an educated person trying to sound uneducated?" Usually it is the latter, so we must then decide "Why is this educated person trying to sound uneducated?"

As someone from the US, I would only say that to someone else if I were joking with a friend. It's not something I would say as a typical answer or to a potential employer. Again, only if I'm very familiar with the person and they know that's not how I typically talk. I'd say it in hopes of making them smile or laugh.

It's likely that you wont be understood. This word is more frequently used in American English. In British English it isn't used so much. So it depends on if the Turkish person has watched a lot of American TV! My advice is to stick to correct language. 'I don't have the time' is much better! Hope this helps :)

Thanks a lot Jerry. That's interesting.