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That's basically impossible to imagine!
You would have to change the dialogue and text files... and that's half of all the files....
Install either the English version of both parts, or the Russian version.....
The registry has nothing to do with it.... It only contains records about the game, and since the English version is looking for English records, it's not surprising that it can't find them!
Regarding changing language versions, that wasn't my idea. And games that rely on the registry are, in my opinion, just an unnecessary headache.
The best option is to install licensed versions of both the game and the expansion. All the patches will be applied accordingly.