Puesta del Sol offers homestays and Spanish lessons on Ometepe. I spent a week with them earlier this year, and quite enjoyed it. The accommodations are fairly rustic, but I would say much better than hotels in the same per-day price range. I didn't try their Spanish lessons, but it sounds like they do at least have prepared materials, so they put some effort into it.
I would recommend that you at least contact them to see if it would work for you. They were a little hard for me to get a hold of (broken email address and no voice mail), but hopefully they have that fixed. If not, send me a private message and I can put you in contact with them.
http://www.puestadelsol.org