I would replace "learn" with "study". A person "studies hard", and does not "learn hard." The next point is a matter of preference. I like "the more you will improve" better than "the more you can improve." The former is a direct cause-effect scenario (the more you do X, the more you will get Y). Saying "the more you can improve," on the other hand, leaves me with the sense that after studying hard, there is only a possibility that you will improve. Of course, maybe this is true! So changing "can" to "will" is optional. So I propose: "The harder you study English, the more you will improve." Otherwise it is great!

Try: The harder you learn English, the more you can improve it. Or The harder you learn English, the more your English can improve.