The best and only language for GTA V mods is C++, specifically Visual C++. For the PC version you can also use C# to an extent, but for the majority of work C++ is the only option. If you don't have Visual Studio you can download the free community edition.
gta sa definatly
Only Available on PC.
there is no mods in gta 4 but for gta sa the spanner icon on your map
mods.
http://www.gtagarage.com/mods/index.php
Gta 4 on xbox360 monster truck cheats
Mods.
You can't. Mods.
mods.
mods.
mods.
im sure that GTA sanandreas has the most mods and the coolest =]