While it's technically possible to add a translation in any one of the languages we have added to TMDB (we don't restrict content), the ones listed in this method are the ones we also support for localizing the website with which means they are "primary" translations.
These are all specified as IETF tags to identify the languages we use on TMDB. There is one exception which is image languages. They are currently only designated by a ISO-639-1 tag. This is a planned upgrade for the future.
We're always open to adding more if you think one should be added. You can ask about getting a new primary translation added by posting on the forums.
One more thing to mention, these are the translations that map to our website translation project. You can view and contribute to that project here.