In BOTH cases the learning is finished. The difference between the two sentences is the time period when this happened. 1. If someone says 'I've learned English in the USA', you'd presume that they are still in the USA now, or have recently returned from the USA. 2. If someone says 'I learned English in the USA', you'd presume that they are no longer in the USA. The use of the past simple ( I learned) tells you that the time period when this happened is finished. In this case, the finished period is the time spent in the USA, indicating that they are not there now.

Learned indicates the action is finished. I would use study instead. In the second sentence "I studied Engilsh in the USA." that as Past Simple indicates that you studied in the United States and now have either stopped studying altogether or stopped studying within the boundaries of the United States. You either stopped studying, stopped living in the united states, or stopped both. So the implication with LEARNED is that you went to the United States and learned English. The same way you might go to Mexico and watch a soccer match "I watched a soccer match in Mexico." The first instance is present perfect (Have learned). It is unspecified time. So for study I have studied implies you are continously doing it, you started sometime in the past and have kept going. Because Learned implies a finished action, you are saying at SOME point in the past you learned English. The best differentiating sentence I can think of is for a movie "I have read that book before." "I read that book yesterday." So how do we answer WHEN for these sentences? One happened sometime in the past (BEFORE), and the other happened exactly YESTERDAY in the past. The second one suggests you can recall the exact time it took to learn english,

There are differences in accent. A few words are different but not a lot. Idioms are a little different, I can almost always understand an American English idiom even when I've never heard it before, but I can't in British English.

It's very similar, have a look here http://ell.stackexchange.com/questions/9532/done-vs-have-done