Sometimes copying files isn't enough. The game still thinks you are in Russia/Germany/etc. You must edit the Windows Registry.

Do not open it in Adobe Digital Editions, Calibre, or any e-reader app. Do not rename it to .exe.

Change the file extension manually.

If you see folders like data_win32 and files like common.dat, you have the real language pack. If you see OEBPS and content.opf, it is a real eBook.