Hi! Well I'm Mexican and I surely have barriers understanding the language. However, I think it's amatter of use. For me both of them are correct. The difference is that International Sales Manager refers to a person who manages sales all over the world. On the other hand, I suppose Foreign Sales Manager is the person who is not native of the country in which something is sold, but that it's not necessary international. e.g. If I go to Italy to sell something, I'll be a foreigner in your country.
I hope this may help you a bit. As I said, I'm not native so I may be wrong.