UhuruNUru: When I installed the game, even though I selected English, the game starts with Chinese language selected.
This may only affect the offline installer version, as that's where I made the language selection.
Seems to me as if the installer installs all language files, instead of the choice I made
It will work the same, for Galaxy installs, if they have the problem

I can't find any setting to change this, so I tried the fix below, and it worked for me.
Should work for any language, as you remove all but what you want to use.

Navigate to your game folder, and open the "packs" folder
You can make a backup archive of the files beginning with "loc_" files if you want.

For Voices you have only two choices, English or German, delete the voices file you do not want
This may actually be better if you want, for example, german voices, and french text, as installer gave no voice choice.
I suspect, you only get german voices, with german text, if installer is working.
loc_eng_voices.spk
loc_ger_voices.spk

Text has more choices, and again delete all the languages you do not want.
loc_chn_data.spk
loc_cze_data.spk
loc_eng_data.spk
loc_fra_data.spk
loc_ger_data.spk
loc_ita_data.spk
loc_kor_data.spk
loc_pol_data.spk
loc_rus_data.spk
loc_spa_data.spk

You are left with one voice file, and one data file, and the game is forced to use them
English, in my case is what I wanted, and what I get in game
loc_eng_data.spk
loc_eng_voices.spk

It should be obvious, but only delete files starting with loc_
You need all the other files. I'm left with
data.spk
datapc.spk
datapchighres1.spk
datapchighres2.spk
datawin32.spk
loc_eng_data.spk
loc_eng_voices.spk
region.spk
sli.spk

Plus the "sli" folder.

That's it, game starts with the only language files it has.
GOG installer seems to be broken for this one, as it seems only the language selected should have been installed.

{Edit Added}
Anyone whose posted a review complaining about this, please alter it.
Games not at fault here, GOG's offline installer is.
Not a single one of you posted in Forums, as this is the first post.
I know the game forum was late appearing, but it did have a launch thread in General; Forum, and someone there posted a renaming file to Chinese fix, which works, but only language user has chosen should be installed.