I'm from Poland, and I get German/Deutsch. Checked in Opera and Chrome, before and after deleting paypal/glbasic site cookies.
What more interesting, once, after login to paypal account and logout I have got that checkout page in Polisch, but next time again it was in German

and still it is..
There is 'lc' parameter for setting user language:
The locale of the login or sign-up page, which may have the specific country's language available, depending on localization. If unspecified, PayPal determines the locale by using a cookie in the subscriber’s browser. If there is no PayPal cookie, the default locale is US.
but it isn't set on glBasic site so everyone should get his default language or at least US English.
Strange problem..
Maybe you should force some lc with info gathered from php user-agent or something like this..