What do I do if a professor at a foreign university doesn't respond to my emails?
Professors directly teaching might have some obligation to answer your email (some university have quality of service policies, ensuring that students receive feedback within a given number of working days), if it's related to their teaching. 

Professors from partner universities might fall under the same obligations, if there is a mutual agreement. However, professors from other university have no obligation whatsoever to answer you. 

Professors are usually very busy, among other things dealing with their own students, they might not have the time to deal with other students. If you are asking about a proposal and you do not receive an answer, you should assume by default that they are not interested. There is no much you can do, apart from contacting other professors, or asking your own professors to contact them.