What do I do if a professor at a foreign university doesn't respond to my emails? I want to know about appropriate actions to take when not receiving an e-mail reply from professors after 2 to 3 weeks of sending an e-mail.

When writing my e-mails, I try to highlight questions and important points leaving other details in normal style to make the reader quickly understand what am I asking for. Additionally, I start the e-mail with Dear Professor , I conclude with a respectful salutation form and I proof read the text of the e-mails multiple times. I am sending e-mails to professors asking them for a graduation project proposal.

E-mails are sent to professors responsible for European or North American funded research projects.
Professors to whom I sent e-mails are from various institutions and countries (USA, Canada, Switzerland).
The same problem is faced in previous occasions when I needed to send e-mails for various purposes but I don't receive replies except for one or two times.
I tried twice sending reminders after a reasonable time (one to two weeks) of sending the first e-mail but no response is received!

N.B: if it's helpful to mention, my field is not covered by any research lab and not taught in any university in my country.
And this is what makes the problem more complicated!
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.