You can only use your "Delta miles" in Delta's Skymiles programme...you cannot use these miles to book anything in the Flying Blue programme, so you have posted this question in the wrong place.

Delta allows you to use your Skymiles not only to book awards on their own flights, but also to book awards on partners including AF and KL. However, given the huge demand for travel, and given that December is only 4 months away (these seats have been open for booking since December 2021/January 2022), it may be that Delta no longer has any availability on AF or KL flights on the specific dates that you require.

So, to summarise: yes, you can use Delta miles to book flights on KLM and Air France, but you must book using the Delta Skymiles programme, and you need to be flexible with your dates and expectations (and it's always best to book soon after the flights become available, almost a year in advance).

There is no point ringing Flying Blue or asking to speak to anybody in Flying Blue, Air France or KLM; they cannot accept your "Delta miles"; the only programme that uses "Delta miles" is Delta's Skymiles, so it is a Delta Skymiles agent you will have to talk to to use these miles. And it is not possible to transfer "Delta miles" over to Flying Blue. The only way you can book any flight (whether on Delta, on Air France, on KLM; or indeed on ANY airline) using your "Delta miles" is to do so using your Delta Skymiles account or by contacting Delta Skymiles customer service. You cannot contact Air France, KLM or Flying Blue about using your "Delta miles", because they cannot accept them, nor do they have any mechanism for allowing a booking to be made using "Delta miles".