Using the uc_paypal module, the "PayPal Website Payments Standard" admin page has the setting "PayPal login page language" from which we can choose a 2 letters localization code.
Our place of business is in Montreal, where language is French (FR), and country is Canada (CA).
Setting the "Paypal login page language" to FR sends the following code to PayPal: lc=FR
And the address country field, filled by the customer on checkout sends the following code to PayPal: country=CA
The way PayPal treats this info results in the pre-populated country field of the PayPal payment page to be "France" !
We would hope that PayPal would not override the country code with the locale code, but that is what their page programing does.
My guess is that in cases where 2 letters language code doesn't match the country code, PayPal's page programing would need a 4 letters format localization code to distinguish language from country. In our case this would be "fr_CA" for French Canada.
Otherwise, my understanding is that the FR locale code defaults to the language support priority 0 which is fr_FR (French France)
See this PayPal developers info page:
Country Country Code Language Support Priority Locale Code
CANADA CA 0 en_US
CANADA CA 1 fr_CA
FRANCE FR 0 fr_FR
FRANCE FR 1 en_US
This would have to be verified with PayPal, and the uc_paypal module would need to support 4 letters localization codes.