I travel extensively in these areas, in fact, I leave for Guanajuato Mexico tomorrow for eight days.
El Salvador and Honduras are no-go due to violence. Honduras is worse, but there's not much to see in El Salvador, so I throw that in there as well.
Guatemala has issues but for language schools it's pretty good. Not sure you'd get much good training done there.
Nicaragua is really quite nice, people are very poor, hot, hot, hot.
I don't go to Belize or Costa Rica -- too touristy.
I've only been to Colombia in South America, I can't comment much otherwise there.
Central Mexico could be a great place. Lots of great cities, some at decent altitude. Relatively inexpensive. You will see people running. Violence concerns are way overblown.