If you know c++ you could basically roll right into any of those languages I named above in under a week as well.
It is sort of difficult to do anything worth doing on the web unless you have experience with at a minimum Javascript, a serverside language(php, ruby, python, c#, whatever. You could maybe do nodeJS but if you are doing that you are probably already rolling deep in knowledge anyway) and have some experience with some kind of sql like database language. I'm not even counting html or css which aren't real languages but are still things you need to have some understanding of. Also there is a good chance you have a basic understanding of setting up apache and a linux server in a cloud environment.
If you're doing any of this stuff you have probably tinkered with mobile and maybe have some obj-c experience or at least java on android.
Everything is just so tied together these days unless you are just like, a day job programmer you probably have experience with a ton of languages and could get some code going in a bunch of others very very quickly.