When it comes to the older programming languages - COBOL, FORTRAN, C, various assembler languages, PASCAL, LISP, ADA, I've programmed in every single one of them.  The concepts are usually very portable, it's just a matter of learning specific semantics.  Conditionals, functional programming, loops, etc, change in specific implementation, but not in concept.

Frankly, I've always personally interpreted a developer with experience in just one or two languages as, at best, woefully inexperienced.