In an ideal world, you'ld know only one language; one that was expressive, powerful, high-level, and easy to read. Everyone would use it. Performance wouldn't be an issue, because the language would have a mythical self-optimizing compiler that could generate faster code than the best hand-tuned assembly. It would determine intent, and re-write your algorithms to equivalent, faster algorithms on the fly. It would sort lists in constant time, chop onions, and make jullianne fries.

While you're dreaming, dream big! :-)