Hope I can help a bit:
Malaga: lots of English speakers, very touristy, but it has it's charming side as well. Speaking Spanish or English all depends on who you get to know. It has a big, well connected airport.
Granada is a fun town with that in-between sized feel. Lots of university students, good night-life
, lots of mountain tourism (hiking, skiing), mostly Spanish speaking, but you could find a group of English speakers to hang out with if you wanted. Airport: go to Malaga or Sevilla
Seville (where i've been living for the past 8 years): very Spanish city if you avoid the tourists. Most of the city doesn't speak English at all, so you'll be forced to speak Spanish. You could find some English teachers or exchange students to hang out with and speak English, but it's very easy to meet Spanish people and get integrated. Good airport with Vueling, Ryan Air, Air Berlin et al connections to Germany. Seville has it's charming historic center, monuments, decent nightlife and a small percentage of foreigners living here so that the authentic culture feels pretty unspoilt. (similar to Granada, both better than Malaga).
Good luck!
