This game, originally, was multilanguage. In the GOG version miss file "LanguageSel.exe" that allow to select language to start. There's the possibility to including this file in the GOG edition of game?

I think you should ask this question directly to GoG staff in the support area of the site.

http://www.gog.com/en/support

I think they can anwser this question better than anyone here (I think...)

There is it, i uploaded it for you. I don't sure it will be working, but give it a try.
Make a backup of the enclave folder, you never know what will happen, and after that extract this into the folder you installed the game. Then run it, and select the language you want.

Here's the LINK