There really is no such thing as "international criminal law".
While there are some international criminal court cases at The Hague in Amsterdam, they are very rare (like 1 every few years....if that). And those cases are handled usually by European lawyers with decades of litigation experience. If this is something you really want to pursue you will NEED to graduate from a top 10 law school (Harvard, Yale, Columbia, etc) near the top of your class, secure an elite job, do that for 30 years, and MAYBE you might be able to work on one international criminal case at some point in your career.
And when you are in law school, you don;t have a major, you don't pick a single area of law to study. You will need a well rounded legal education, and you will learn a bit about all aspects of law. Your school will probably have only one class in "international law", and 80 % of the material will deal with international business deals and contracts, with a small part of the class focusing on treaties.