So are you looking for adventure or a paying job? Seems like you're not much into luxury and just want the experience, so why not try volunteering - wwoof, helpx, whatever - as long as you get room and board. If people are paying to learn English (and you're getting paid to teach them), they'll expect some kind of certification. But if you're volunteering, it's not as though you're ripping them off by not being certified. You're more literate than lots of people on this forum, and you seem to have the right attitude.
BTW I was in Nicaragua 20 years ago ... absolutely beautiful!