As development is done on multiple OS, there are also multiple IDEs used.
On Windows, it's best to use Visual Studio to do the C++ development. Apart from that, IDE choices are pretty much free, and even simple text editors work quite good. I normally use gvim with some adapted settings (like using tabs instead of spaces), and it works well enough for me.
About the most used languages. Generally it's C++ and JS. C++ is used for the parts that need to be fast (actual rendering, pathfinding, ...), JS on the other hand offers the possibility to be run without compilation, and thus is ideal to be used for game logic, and to be adapted by mods.
The other languages used are mostly because there was an interesting library available that could be used. Like a chat library in python for the lobby, a compilation library with lua, ... These parts generally don't belong to the core game, and could rather easily be replaced by other tools in other languages.