Yes, you can download subtitles separately. They are very small ( usually a few kilobytes), basically simple .txt ( or .srt ), but you need to find ones that match exactly the version of episode you have ( framerate, length etc). Most players load them automatically if they are named the same as anime episode, otherwise just look for "load subtitle file" option in player menu. As to where you can find them, i can't help you, since i don't know spanish, but spanish anime community is quite active so look around spanish anime sites and you should find them.