7 – The Klingon language

It's a constructed language for a fictional race of alien warriors, designed to seem as alien as possible to as many people as possible while still being usable by human actors. It manages the trick pretty well, yet if you're more dedicated than I am, you can actually learn and use it on your own. Even just learning a little bit about it, though, is a fascinating glimpse into what natural languages theoretically could be like, but aren't.