I'm spanish, and I've the original CDs in spanish, though I usually prefer the original voices in this case I really like the spanish voices (And it's easier for me to understand what they're saying, cause there are not subtitles).

I know that I can change the MIX archives from the GOG directory with the spanish ones, but I don't find them. I'm becoming crazy looking for the MIX archives. By the way, I play on a Mac, if it is useful in some way.

which game are you talking about?

for LoL1, you can open the Game.DAT file with an archiving app like winrar, in there are the language files

for LoL2, the .MIX files are located in ...\GOG Games\Lands Of Lore 2\DAT
you have to overwrite all of them iirc, you also need to overwrite the following files in the main directory with the ones from your language version: CREDITS.TRR, OPTTEXT.TRR and LOCALLNG.MIX

for LoL3 i will have to look into that first.