From MVP Ronnie Vernon:
You cannot change the default language for the Operating System, except
under certain, special circumstances.
You can change the language using the Windows XP MUI (Multilingual User
Interface) but only if you have the English version of XP Professional
and only if you are a corporate customer or OEM. It is only available as
a Volume License purchase.
Other than this, the only language options are the IME (Input Method
Editor) which gives you the ability to change the keyboard to input a
foreign language in a text document. (Goto Control Panel-> Regional and
Language option -> Language tab
check the language you wish to use, Follow instruction (Need XP CD)
reboot the PC.)
So to answer your question, you will need to purchase a separate English
language version of XP.
Multilingual Features in Windows XP Professional:
http://www.microsoft.com/windowsxp/pro/techinfo/planning/multilingual/lic
ensing.asp