Replace the Japanese.zip locale with your English.zip locale from gLunia.
1. Copy the English.zip locale over to the Lunia\Locales\ folder
2. Make a copy of the Japanese.zip locale somewhere else (in case something goes wrong)
3. Rename the English.zip to Japanese.zip in the Lunia\Locales folder
4. Run jLunia
Everything should be in English except for the stuff we don't have (which will be displayed as Global.UI.etc.etc.)