I own a retail copy of Dungeon Keeper, and the game was multilanguage back when it was released. So why remove the speech files from all other languages other than English? Are you going to fix this?

No chance, i fear. Multiple languages means multiple publishers in the different countries, so GOG would have to have a deal with them too... Some publishers are retained the rights to every language version of the games (so you have some multilanguage games here on GOG) but not all...

Yes, but as far as I know, Dungeon Keeper was published by EA in all countries, at least in Europe. So the rights should not be a problem - they belonging to EA, who authorised the game in the first place. Or am I wrong?

in theory you're right, but the translation might belong to another company...