No. It's Diable and Ifrit Jambe respectively.
Akuma and Majin are the Kanji Oda chose to use for those readings. In the anime they are pronounced that way, and in the manga the furigana is literally Diable ( ディアブル ) and Ifrit ( イフリート ).
This linguistic device is called Ateji (当て字), but in the specific context of manga and anime, it is more precisely known as Gikun (義訓).
So, if Oda decided to name it Surtr Jambe, for example, it would be pronounced (スルト), and the Kanji would probably be something like 巨神 (Kyoshin = Giant God).