Hi, native Spanish speaker here! Try these:
-Pokémon Xenoverse, by Weedle Team - hosted over at the Completed Projects sub-forum. Originally in Italian, now translated into English. Easily one of the best long-form fangames I have ever played; if you are looking for one that sticks to a traditional Pokémon experience, this one is for you.
-Pokémon Argentina, by Sky and Geo. Argentinian Spanish. A Pokémon game set in a region based on Argentina (my home country!!). It's part serious Pokémon experience, part meme game; I recommend it mostly for the latter because there's a lot of Argentinismos you've /got/ to see for yourself.
-Pokémon Panda, by Jaizu. Spanish only. Blasted hilarious meme game featuring a Detective Pikachu!
-Psyduck Hunt, by Kyu, Kid, Clara and Caeles. Spanish only, but again, easy to read. A Duck Hunt clone featuring Psyduck and some friends. One of my favourite short-form fangames - we need more arcade-style games!
-Wooper Collector, by Jesswishes. Spanish only, but has minimal text anyway. Still in beta and thus has a lot of room for improvement, but it looks pretty promising!