I own the 2 Far Cry games and 4 PoP games and did a quick check to see what I could find without actually installing them full games as that's way too much effort. Just clicking Install and looking at the available choices yields:
Far Cry - English
Far Cry 2 - English, German optional
Prince of Persia - English
Prince of Persia The Two Thrones - English, French, German
Prince of Persia Warrior Within - English, French
Prince of Persia Sands of Time - English, French, German
If any of the games have more languages available than that, they do not show up in the installer screens in Galaxy.
GOG often tracks down additional languages however if someone shows an interest and they're able to track down the additional content and get legal license to use it - however that is not always possible. Keep in mind sometimes a 3rd party is hired to localize a game for some other country/language and they may retain distribution rights in that country and GOG may not be able to obtain that, however they very well might be able to also.
You might want to ask JudasIscariot about that perhaps, or some other GOG employee here in the forums. You could ask support too, but they're more geared towards support than towards the tracking down of potential future additions.
The GOG store page, GOG support, and MaGOG all most likely get their data from the exact same database, so will likely generate the same results I imagine, and one can just look at the store page for that without extra 3rd party help. Sadly, it looks like you're out of luck for Spanish for those specific games for now from what I can see.
It might be possible to download additional language support from some random website out there that may be a less than authentic source of data. There are localization enthusiasts out there from what I understand whom try to localize games to other languages or possibly gather existing localization together, although that may not be legal likely so beware.
