Something I have forgotten on the specific topic of DeusEx: it may be worth a try to get the language-related files from English players.
TTLG is a rather nice community for the games from Looking Glass and their splinters, and someone there may be able to upload the appropriate files somewhere. If you are lucky, Deus Ex 2 still has the original speech in the translated Versions, as Deus Ex 1 did.
Applying the English patch can do the trick for some games, but no promises here. If you are truly lucky, they will have made some international patch working on all Languages, and allowing you to switch the game back to English. Even language-specific patches can trick a game sometimes; whether it will work with Deus Ex is another matter. I don't have the game myself, so I cannot be more specific here.