ysvry: nice your trying to help, try installing the spanish game , run it once then try installing the english version ontop see if that works too.
So did you first install the Spanish version, then installed the English version on top of it (in the same directory), without uninstalling the Spanish version first? I'm not a doctor or anything, but I'd suggest you should not install different language versions on top of each other.
When you contact GOG support, I propose you explain very clearly and in a very detailed manner, what steps exactly you have done so far, and what exact error messages you receive at which points.
I've installed and uninstalled lots of GOG games numerous times (with the installers, not using GOG Galaxy), and the only problems I recall facing:
1. I tried to install a game whose installer was corrupted (the .bin file). Since it was a smaller game, it didn't automatically perform the integrity check before installation. like the bigger games with several .bin installer files do. I ended up with a partial installation that couldn't be uninstalled. I fixed it by redownloading the game installer, and installing it on top of the partial installation. Then I was able to uninstall it too.
2. A couple of times I've done an user error. I wanted to transfer the game from one PC to another, so I figured the most foolproof method (save games, game profiles, settings etc.) is that I install a fresh game on the other PC, and then just copy all the game files from the first PC on top of that installation.
What I didn't take into account was that on the first PC I had installed the game on a different drive and/or directory than on the second one. Uninstallation failed at that point because on the new PC the uninstall script was apparently looking for the game or uninstaller tool on the wrong location (the place where the game was in the first PC).
I don't recall how I fixed it. Maybe I just deleted the install game directory and then reinstalled the game from scratch to the same directory.