AP doesn't have an absolute variable for this like some of the other Classic APIs, however as a workaround, you could add the country code to the redirect URL for some languages. For example, the below URL will display in German due to adding the /de/.
https://www.paypal.com/de/cgi-bin/webscr?cmd=_ap-payment&paykey=AP-xxxxxxxx
Keep in mind that not all countries will display the corresponding language. Another thing to keep in mind is that other factors could override this as well such as default browser language, buyer location, and browser cookie.