If I were to become a historian of rome/greece, would I have to learn latin and greek? Is it really necessary? Haven't most texts been translated to English?
This is entirely dependent upon the school where you choose to study and what requirements they have in place. There are many universities out there that offer classical antiquity degrees which require some study of Latin, Hebrew, and/or ancient Greek. There are also plenty of universities which do not have such strict requirements.
If you are truly dedicated to the historical study of a particular era, however, it would be nothing but beneficial to have some understanding of the languages and scripts pertaining to that area.
Keep in mind that anyone who studies primary source documentation translated into another language is reliant on a third party for the translation. This isn't necessarily a deal breaker, it just means you have to be careful about what translations you use.
There was actually a super interesting discussion on this very topic a couple years back. As well as the distinctions between ancient historians, and Classicists and the demand (and roles) for both.