Hello !
I think that change the language is very easy as far as you have the correponding files. Let me explain what I did yesterday.
Fan of Ceasar III, I bought Pharaoh in french when it came out twelve years ago and my young daughter felt in love with it too. At these old times we were in Windows 98. Later Cleopatra get out, but after the 50 € of Pharaoh, I admit that I did download this extension with the related patch to make it works. Later the whole stuff was transfered on Windows XP, still working. Today my daughter have her own PC, we're both under Windows 7 and I discovered that the Cleopatra extension and its patch were not working any more on all configurations. Since then I succeded to have it working on my PC, but not on the one of my daughter and she was very deceived because she was still playing with Cleopatra from time to time... and these times seemed over.
So two days ago I bought the whole stuff (again for Pharaoh) for my daughter on GOG and it works fine on my PC... but that is the english version, which I stupidly did not notice when I bought it on GOG. So what did I do ? I installed my old official french Pharaoh in another directoty than Pharaoh Gold, installed the Cleopatra extension without the magic illegal patch... and simply put the pharaoh.exe file from Pharaoh Gold at the place of the one coming from the old french Cleopatra... and it works ! I have now a fully french LEGAL Pharaoh Gold !
What I think is that if to replace pharaoh.exe with the one from GOG in a 2000 french release is sufficent to obtain a full french Cleopatra working game, it must be possible to pack all the files that are specific to the french release and deliver them with GOG's Pharaoh Gold in order to obtain a fully french Pharaoh Gold. When I will have time, I will do the trial in order to be sure (replace english files by french files into Pharaoh Gold installation).
And so the final question is for GOG : do you have the rights to release a french version of Pharaoh Gold ? If yes, I think that to pack it will be very easy. And as a consequence, it should be the same for a german version (but I don't have the files).
Sorry for this long post, hope it will be useful to somebody else.