That's a tough questions. In general in the scientific field Python is popular, so learning it definitely does not hurt. For real-time stuff C++ is also useful.
So there really isn't a "best" language, I'd just go whatever feels best for you. The next KUKA|prc (no exact ETA) should make external scripting via C# quite a bit easier as well, but we haven't decided yet by how much we can open it up to users.