Personally i would have thought RT in standard english would be the best bet. If you transmitt initial calls in Spanish, then the controller could throw a lot of clearances and the like at you in Spanish which i would have thought would need a greater understanding of the native language. Remember KLM 1974 in Tenerefe? Wasnt quite sure of his clearance, so initiated the take off roll anyway. Meanwhile Pan am is trundling down the runway as he thought the tower must have meant exit C4 not C3, and then about six hundred people died when they collided! I agree it is good manners to attempt the host country's lingo, however i suggest practicing it in the bar and buy them a drink!