The below message is in English and Polish. If it's ok with the mods, please maybe make this topic a sticky or suggest where I should link it so that more people can find it easily.

I'm hoping GOG will soon add other language versions to both Gothic 1 and 2 beside English and German but in the meantime I've come with a small work-around.
I extracted the files containing the videos and audio dialogues in Polish and made them into a .rar. You need to unpack this to the directory where you installed your Gothic from GOG and agree to swapping the files which are already there with the new ones.
I haven't been able to locate a file with all the written text that I could then swap for the Polish version - if anyone knows where it is, please let me know.
Also, I'd be glad if anyone could test this properly, maybe check if it doesn't cause any problems when being used with any tweaks, patches or mods.

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Link: http://www.filehosting.org/file/details/482812/G1_Polish_dialogue_video.rar

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Mam nadzieję, że GOG w końcu doda więcej wersji językowych do Gothica 1 i 2 poza angielską i niemiecką. Póki co wpadłem na pewien pomysł.
Wyciągnąłem odpowiednie pliki video i audio z polskiej wersji i spakowałem je do .rar. Należy wypakować je do folderu, w którym zainstalowany jest wasz Gothic z GOGa, podmieniając te, które już tam są.
Niestety, do tej pory nie znalazłem pliku, w którym zapisane byłyby wszystkie kwestie jako teskt - jeśli ktoś wie, gdzie takowy jest, proszę o kontakt.
Nie testowałem całej gry z tymi plikami, ale wyglądało to nieźle (poza napisami po angielsku). Jeśli możecie, dajcie znać, czy nie powoduje żadnych problemów z tweakami, paczami czy modami.

I agree with a request for polish in this game. Gothic series, espiecially 1-3 are almost like Witcher to us, players from Poland. We can play it in english, but it would be very different from our "old" experience. And the guys responsible for the voices in our local version... they did a great job. It's part of the feels, to listen "that" Xardas, "that" Lee, "that' Diego and so on and on.

I know and it's an option if my CD/DVD version fails me... But I would prefer it here, just like other games... But, who knows, maybe they will manage to get it, just like some other games? :)

I tried buying a game from them once but couldn't - they restricted buying to only IPs from Poland - that's not where I live unfortunately :(
Also, I prefer GOG as they're not just a web shop with games but people with passion :)

Try this:
http://forum.worldofplayers.de/forum/threads/1391973-Release-Gothic-1-%C2%97-LangPack-%28Latin%29-%28ENG-%29

I can't get the thing from MaGoth's post to work properly. It says that after installing I should have seven files in the Gothic\Data directory, but I can only see three: anims.VDF, meshes.VDF and worlds.VDF.

EDIT: I tried it one more time and now it worked... I have no idea what I was doing wrong before.