Scala is, if memory serves, a Java derivative, hence why its useable for Minecraft modding. Theoretically, you could use most any language, so long as you had something that could compile it into JVM bytecode. If you've got either the tools or the time, you might could write mods in C++, C#, Pascal, Ruvy, Clean, or even Brainfuck, if you just wanted to. Again, just need to compile it into something the JVM can understand.
(Or, alternatively, make a hackpack that'd allow the JVM to run the code, anyway, even if its actually compiled. Though, you may need to release 3 versions of your mod for the 3 main operating systems. Or just do like I'd do and release for Windows anyway because the rest don't matter.)