Turkish isn't here, yet, because Duolingo is only a year old, and the site is still being improved.
Additional languages will be added by users, but Duolingo hasn't uploaded that section to the website, yet.

I think Duolingo is trying to create a system where it will be possible to build languages ourselves, instead of having to wait for "central command" to do it. It means we must wait longer initially, but hopefully when the feature becomes available, a massive number of languages will become available.