It's great that many games come in localized versions these days. However, there can be several problems with those:
1. Without wanting to bash anyone, some localisations are simply bad. Doesn't have to be the case with GC3, but...
2. Even when it's not bad, with multiple patches, addons and whatnot the potential for messing something up in the process rises. IIRC in some cases (non-Stardock games) the main game was localized, while some of the additional content was not, or not that well, leaving people with a strange mixture of the original and some half-baked localized version. In some cases modders did improve localisations on their own, but this may take time, or simply not happen.
3. Even with everything done well, some players may prefer playing the original, since every localisation remains a translation and some things might simply work better in the original language (humor, word play, whatever...)
Please give people the option to install/run the game in the original English language, or in a localised version when available. IIRC this was not possible when you bought a localised GC II (which I did).
So in short, please allow people to play in English if they want.