There is a solution, you can use this patches that where created for the steam version, but by copying manually the files on the correct folders, and by some little editing of the monkey3.ini file this also works on the GOG version, here are the links to the steam posts with the download links. I got this to work for the spanish version

France: [url=http://steamcommunity[DOT]com/sharedfiles/filedetails/?id=1338777098]http://steamcommunity[DOT]com/sharedfiles/filedetails/?id=1338777098[/url]
Italian: [url=http://steamcommunity[DOT]com/sharedfiles/filedetails/?id=1339702271]http://steamcommunity[DOT]com/sharedfiles/filedetails/?id=1339702271[/url]
Portuguese: [url=http://steamcommunity[DOT]com/sharedfiles/filedetails/?id=1339630364]http://steamcommunity[DOT]com/sharedfiles/filedetails/?id=1339630364[/url]
Spanish: [url=http://steamcommunity[DOT]com/sharedfiles/filedetails/?id=1339957793]http://steamcommunity[DOT]com/sharedfiles/filedetails/?id=1339957793[/url]
German: [url=http://steamcommunity[DOT]com/sharedfiles/filedetails/?id=1340194741]http://steamcommunity[DOT]com/sharedfiles/filedetails/?id=1340194741[/url]

for the links just replace the [DOT] for an actual dot, had to do this because it didn't let me post links

All of that won't be necessary anymore, since GOG rolled out an update on GOG-Galaxy which adds several other languages (German, French, Italian, Spanish) to the game. I assume the manual installers will be updated shortly as well.

Did I miss something? I did not receive this update on GOG Galaxy.

Looks like it was an accidental early release and they rolled the update back. Sorry.

It's live:
Language Support Update (1 June 2018)
-Added language support for French, Italian, German and Spanish.

Just change the language in GOG Galaxy under 'Settings'.

It seems it appeared on Steam. I guess we'll get it too soon.

[Edit]
Wrexter beat me to it :)

Little off topic maybe:
I have a scan of polish manual for CoMI and I would gladly share it on GOG.
Do anyone know if theres a way to contact GOG staff for such a matter?

Check under support and be prepared to wait.

Thx I can wait :)