Documentation:
http://www.ubercart.org/paypal
also: http://www.ubercart.org/forum/support/3265/paypal_website_pmts_standard_checkout_page
Some definitions:
- PayPal Website Payments Standard (payment method)
is a payment method used in conjunction with the UC checkout system - PayPal Express Checkout and Google Checkout (payment methods)
are designed to totally circumvent a site's own checkout system (checkout disabled by default) - PayPal Website Payments Pro (payment gateway)
a payment gateway, where the user user will not leave the Ubercart site... (US only [091130]). But it is enabled by default in the Payment gateways page.
So: Using PayPal Website Payments Standard, everything else is not enabled /gets disabled.
Multilanguage
Multilingual variables:
Paypal login page language: uc_paypal_wps_language (table: variables)
More: a comprehensive list of variables at http://www.ubercart.org/contrib/8319 in the tar.gz's module file. Since I already have a few of those variables specified in the settings.php file, I have not installed this. I have no idea if it's a risk or not, but I want to avoid any potential duplication issues.
Found unrelated but maybe important: PayPal login page langage is incomplete