I don't think DL lets you change it. I did a roundtrip search ex-US which will use US point-of-sale inventory for both outbound and return. If you do a one-way or roundtrip search from another country, it will use that country for point-of-sale inventory. ITA Matrix lets you specify it, but I couldn't use usual tricks to bring it up on delta.com. Third-party booking sites and partner AF/KL websites all look to use US point-of-sale inventory if booking on their US websites, so that looks to be only option.