Prague and Brno would be the two options I'd consider seriously. Prague definitely has a larger English-speaking population, but Brno is not far behind and is generally cheaper. Prague is exceptionally beautiful, however, and so if you are only going for a few months it might be worth the cost (although travel from Brno to Prague is not terrible on the freeway).

It's worth learning as much Czech as you are able to, but you would be fine managing on your own for a few months, especially if you can befriend a local who can help you get on your feet when you arrive (tips on currency conversion, places to eat, places to avoid, setting up a transit pass, etc.)

Life in Prague if you're coming to visit is quite great. I love every minute of the almost two years I was able to spend there and around the Czech and Slovak republics.