I've found the "drug" for foreign languages , by letting them watch certain cartoons when home goals are met (good behavior etc), with the caveat they can only watch with audio that is a specific foreign language.

The next step is the hardest one. Once they read, I can probably migrate to a 4th language, and leverage RPGs for that purpose. Or I can branch out to programming like you say, but that may be much more difficult.

You need to have a partner that is supportive of this. If you don't present a united front, any (learning) policy will soon be forgotten.