I've searched in the documentation of PayRequest message and there is no option\property there to set the paypal page default language .
currently the page is shown in english and I want it to be displayed in hebrew.
I did try to add this line in my form :
<input type="hidden" name="lc" value="he_IL">
though it seems that this doesn't work when using Adaptive payment .
The RequestEnvelope has only errorLanguage property which is not what I`m looking for ( and its only option is en_US)
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.