I think it depends on the makes no? or on countries as well? So we must adapt the names case by case between the three possibilities DeLuxe; De Luxe and Deluxe...
On Simca for instance the term is De Luxe

It probably depends on the makes, yes.
I think that I'll make an automatic replacement based on the make, using this topic as reference. So any info for all makes is welcome Thanks to those that already gave some info.

It´s more complicated than that, it even changes over the years depending on what was most in fashion at the styling departments.
For Chevrolet it´s De Luxe at least on the pre-war cars and about 1950 they tend to use DeLuxe. I have no reference to show for the moment though.

Here some samples for Chevrolet from Mc Lellans automotive, the best I could get for the moment. They´re rather small but I think you can see fairly well.

1939: Master De Luxe
http://mclellansautomotive.com/photos/B30484-2.jpg

1940: On the cover SPECIAL DELUXE
http://mclellansautomotive.com/photos/B37368.jpg

Inside the brochure Special De Luxe
http://mclellansautomotive.com/photos/B37368-2.jpg

1949: Fleetline De Luxe in the commercial text and the badge on the car DeLuxe
http://mclellansautomotive.com/photos/H2393.jpg

Browsing through Plymouth and Ford gave a similar picture, the word changes back and forth but mostly it´s De Luxe or at least hard to determine wether it´s a space or not.