When a purchase order is sent as pdf to a vendor as attachment in an e-mail, the filename of the pdf has the word "Purchase Order" in it. When you set the user language to another language the file name contains the word "Purchase Order" in that language.
When you purchase at a vendor in another country you want to send a pdf which has a filename in their language. The language should be taken from the language setting on the vendor card.
The same is for pdf's for Sales documents