Use 7-Zip or WinRAR to extract the downloaded archive. You should see a collection of .pak and .dll files.
A: Absolutely. Cyrillic text is stored in russian.pak . Overwriting with english.pak replaces all UI strings, including the build menu and troop commands. Stronghold Crusader English Language Pack
A: Yes. Keep the original .pak files in a separate folder. When you want to switch back, delete the English files and restore the originals. Use 7-Zip or WinRAR to extract the downloaded archive