You can add any language that you can find in the IANA registry: http://www.iana.org/assignments/language-subtag-registry/language-subtag-registry.
You can add the language by choosing Add New Language on the project front page (Project tab). Then enter the IANA language code into the 'Add language' field far down in the popup window (see below).
Please note that the project owner might have restricted who can add new languages. If this is the case, you might not be able to add new languages at all.