A polyglot is defined as one who is fluent in multiple languages. This is similar to how a multilinguist is ... uh ...fluent in multiple languages. (OK, who invented two words for the same concept?)
Well, it turns out polyglot applies not only to spoken languages, but to programming languages as well. Thus in the software world, a polyglot application is one built on multiple languages. The term also applies to developer tools that can manipulate such apps.