DuoLingo in Russian; Chinese; Arabic; Let's start with the Languages that Ruled the Ancient World.
This is the BEST forum I have found for taking this APP to the Grade School kids; they are looking for language coaches and could learn a new language every six weeks. I had one student who was High-functioning Asperger's, who taught himself 33 languages in six months, using language dictionaries that he checked out at the local library! I was only teaching him how to say "How Are You" in the dozen or so languages that I had learned from people that I worked with in the Prisons. He took my coaching and RAN with it; eventually he got listed in WHO's Who for High School students as a LINGUIST.