Best place to visit in Spain in August with the intention of practicing Spanish?

Madrid is the obvious option, but in August I'm told it gets ridiculously hot, and is somewhat boring as most people leave and head for the coast on holiday, so Madrid is left being very quiet with many shops shut.

Although Barcelona is cooler, I gather as the primary language is Catalan, it wouldn't be the best place to practice as quite a few people are touchy about using Spanish and would rather a foreigner spoke English than Castillian.

I also considered Northern Spain, but that region also seems to be a poor choice, as Navarre and the Basque region use Basque a lot, the Asturias region natively uses Asturian, and Galicia largely uses Galician.

Of course, all mentioned regions of Spain will speak Spanish, however, I want to visit somewhere where Castillian is the sole language used for communication and the language which people speak natively amongst each other in the streets, however, also I would prefer if it wasn't ridiculously hot in August :P.

However, the problem is I guess, most of the regions that speak only Spanish natively are South of Madrid and thus very hot.

Thanks for any advice.