When a buyer logs into their PayPal account during checkout, PayPal applies whatever language preference the buyer has specified in their own account -- e.g., buyers who prefer English will get a checkout in English, buyers who prefer Spanish will get a checkout en Español, etc.
On the login page itself before the buyer logs into their PayPal account, PayPal will set the initial language according to the buyer's chosen shipping country (if any tangible goods are being ordered).
Unfortunately, PayPal still has not added a localization (language) setting in their system for Mexico, so any buyers who select Mexico in the cart as their shipping destination will see English on their initial PayPal checkout/login page, but once they log into a PayPal account, the rest of their checkout will be en Español if they had already set that preference in their PayPal account previously.
