Well my google-fu is weak. I came up with a few posts talking of the same idea (most were trying to learn european spanish) but no one actually has a guide on how to do it. Now I found an old post for the beta of the spanish client, but it's (of course) in spanish. Haha, what a catch-22.

Me: I want to learn spanish!

Blizzard: Well here's a guide on how to install the Spanish beta!

Me: But it's in spanish?

Blizzard: Yep.

Of course, Blizzard does not need to cater to people trying to learn spanish. However that beta is old and I know the released client is out. Just not sure how to switch over....