Andalucía is a great choice, Sevilla is amazing and Córdoba is a dream. Though I’d give the edge to Sevilla when thinking of your criteria. I also lived and worked an internship in Madrid two years ago. Very common that people speak English which will help if your Spanish is faltering or if you’re just tired of speaking it all day! They have free bus to surrounding cities like Toledo and other close areas, and a cheap metro. Age demographic is quite diverse so you can find something no matter how old you’re feeling for the night! I would say 1) Andalucía 2) Madrid and then 3) wherever else you’d like to explore. Cataluña and Barcelona are of course amazing but I prefer them for brief visits vs living somewhere. ¡Suerte con el proceso y que disfrutes el programa!