Hello,
I have read that all versions from BASS from Revolution Software are delivered with subtitles in different languages.
How could I change the subtitle language if this is true?

Greets

GOG version is English only.

You probably have to download and install the newest ScummVM from here - http://www.scummvm.org/downloads/

You can download freeware version of Beneath a Steel Sky from here (which probably have more languages) - http://www.scummvm.org/games/#sky

How to change a language - http://wiki.scummvm.org/index.php/User_Manual/Configuring_a_game_in_ScummVM#Beneath_a_Steel_Sky

Polish subtitles (fan, unofficial translation) - http://grajpopolsku.pl/downloads.php?cat_id=1&download_id=329 - work also with GoG version.

If you want use Gog version (I don't know if this version have more languages in standard than English) you can (or should) still install newer version of ScummVM - http://www.gogwiki.com/wiki/Beneath_a_Steel_Sky#Upgrading_ScummVM

[Sorry for my English.]