There have been periods in the past where there have been little or no codeshare availability on AF/KL metal within the EU when booking on delta.com. Although when this has happened in the past, they would simply not show options which involved partner codeshare flights. Basically, they would only offer booking options to EU destinations that are served by DL metal. You could try booking your itin as AF/KL coded flights using multi-city on AF/KL websites which have generally not had these issues. You can still credit to DL when booking on their sites (the downside is C+ upgrades can't be processed until day of departure on partner coded DL metal flights).