I'm afraid the site won't be able to pull something like that off. Localized assets of the game have their own copyrights, which GoG would find hard
and expensive to negotiate for each country
However, it should be possible to pick the data files from a localized Myst installation (or the CD itself) and copy them over. Ideally, you should use a Spanish Master Piece edition, but I'm not at all sure there was such thing as a Spanish translation of MP. You could try with the original game, but there might be problems with video encodings, file names or new assets in the game. The encoding issues are more than likely, since the version of Quicktime required by each varied, and QT has never been too good at maintaining backwards compatibility.
You could use
Riveal and any folder comparison tool (or script) to check that the version to merge has the same files as the GoG one, at least.
