it is always "D.", as initial. https://onepiece.fandom.com/wiki/Will_of_D. The word "dee" is just letter name. https://en.wikipedia.org/wiki/English_alphabet#Letter_names Even the Viz media use "D".
And by the way, thought the letters L and R are somewhat matching to the japanese vowel, the correct name is Luffy, Oda himself stated it- https://onepiece.fandom.com/wiki/Monkey_D._Luffy/Misc. (see Translation and Dub issues). It is pure coincidence with the word "luff" https://en.wiktionary.org/wiki/luff . (like Zoro - Zolo). @ Dragou
It is a hypocrity that the official translator (Mangaplus, Viz) make takedown notices, when they actually are destroying Oda's original meaning in dialogues/words; and the fanbase scanlaton like Jaimini/Mangastream kept original words meaning in their translation.