you need a software called "the registry editor" to edit files beginning with HKLM or HKCU.
Go to HKEY_CURRENT_USERMUICurLang and change the hex value to your new language code

find the windows local language hex code

no need to edit registry, changing the current language, currency, date/time display is very easy.
just press the windows button in the bottom then clic installation icon (the first icon), then find the earth icon in the list and click regional settings, there you can choose language, short/long date, currency, time display.

just learn deutch

viper's solution isn't right, this only change locals but not os language, i think you need to flash your htc phone with an english rom version

add arabic  to htc hd2

 how do u use the registry editor ?