Your first part is tautological but worthless. It's never faster to write in a language you don't know versus one you do know.
EDIT: To amend the preceding sentence, it's almost never faster. There are probably languages which people know that are sufficiently in conflict with the problem domain that they could be faster in some other new-to-them language versus the one they know for specific problems.
APL comes to mind as a language that could be faster to learn and use for a particular task, if your particular task involves a lot of pure matrix combination and manipulation.
though, the fact that it wants its own keyboard is a bit of a hurdle :)
EDIT: To amend the preceding sentence, it's almost never faster. There are probably languages which people know that are sufficiently in conflict with the problem domain that they could be faster in some other new-to-them language versus the one they know for specific problems.