i'd like to run the game in french. I saw there was an easy way by installing the ad-on Trial of the Luremaster in french, but at the last part of the installation it says that there is no valable icewind dale and heart of the ...

All you need is a french dialog.tlk patched to 1.42 (which is the Luremaster-Version). Switch your french tlk with the original tlk and all texts ig are french.

Naturally this also works with every other language. Since GoG's version is already patched you probably can't patch it again. And I guess the copy proetection would be activated by the patch.