Answer 1

Well, I would advise you to learn Python and C(++), if you know them enough you may be able to understand many other languages.
What you mostly need, though, will be to search new stuff, try to understand how things work and how they can be used to do something they are not meant to do.

Answer 2

There are many python modules made for pentesting, which makes it an interesting language.
Also, it's quite easy to learn and it has good performances (for an interpreted language).

Answer 3

It depends on what you want to do. For example, if you want to write a rootkit, C will be the language you want.
Most of the time, you don't even need to learn a language, you can just use existing tools. Kali Linux is a Linux distribution with lots of pentesting tools, you should check it out before trying to make stuff that possibly already exists.