The quickest way I can think of is to give the guest account admin rights, log in, change language, log out, put guest restrictions back.
There must be a better way, but, I can't think of one off the top of my head without being on a domain and using group policy.
edit... Just tried... If you are using a version of Windows with Group Policy Editor (Windows 7 Professional/Ultimate/Enterprise) follow this:
Click the Start Orb and type MMC. When MMC is up, click File > Add / Remove Snap-in, Choose Group Policy Object Editor. Click Browse then click the Users Tab and choose Guest.
From here, Expand User Configuration > Administrative Templates > Control Panel > Regional and Language and you can unrestrict these options for the guest account. Also take a look at User Configuration > Administrative Templates >System > Locale Services.