For Sanji power up names and, probably for other characters too (I didn't check everything), Oda use specific kanji that carry a specific meaning but when reading it he uses katakana (that it's like japanese alphabet for foreign words). Usually a japanese vocab could have two readings (on'yomi or kun'yomi) but with Diable Jambe or Ifrit Jambe for example the reading of the technique is totally different from the kanji ones. Of course you can read it the "proper way" but both in the manga and in the anime the readings are in katakana for Sanji's power ups.
- Diable Jambe 悪魔風脚(Akuma Kazaashi) -the first two kanji are compounds 悪魔 (Akuma - Demon, Devil) and 風脚 (kazaashi - wind speed). If you combine these two words you have something like Demon Wind Speed. Now, in japanese the vocabs are words combined by kanji and kanji often times carry different meanings. For 風脚 (kazaashi - wind speed) I think Oda decided to use not the meaning when the kanji are combined but the meanings of the two kanji separately. For 風 (kaze) it means wind as a vocab but as a kanji it can also means "style" or "manner". So if you combine the meanings of the two kanji 風 (kaze - style) and 脚(ashi - leg). You can translate it as "leg style" and combined with 悪魔(akuma - devil) it becomes "Devil Leg Style, or something like that. From there Oda decided to use the katakana ディアブル (Diaburu - Diable) that in French it means "Devil", so it's like using a Japanese kanji for Devil while taking the liberty to pronounce it with the same word in french. The meaning it's kinda the same. 悪魔 (Akuma) = ディアブル (Diaburu) = Devil
- Ifrit Jambe 魔神風脚(Majin Kazaashi) - where the first two kanji are 魔神 (Majin - Devil, evil spirit, genie) and 風脚 (kazaashi - wind speed). Of course here it could be used the same reasoning. 風脚 becomes Leg Style. The kanji in 魔神 are 魔(witch, demon, evil spirit) and 神(god). So if we take in consideration kanji meanings when they are alone it could be like Demon God Leg Style, but 魔神 (Majin) can also be something like a "Genie" or a "Djinn" from islamic religion. So probably Oda decided to use the katakana word for Ifrit (which is a supernatural being from islamic religion culture) イフリート(ifurito) because 魔神 can also be considered as "evil spirit" or "genie". So it's like 魔神(Majin) = イフリート(Ifurito) = Genie, evil spirit.
So we can say that behind the names there are some meanings and words play.
Devil Style Leg becomes Diable Jambe where it is used the french word Diable for Devil.
Demon God Style Leg becomes Ifrit Jambe where it is used the arabic word Ifrit for Demon God (Or Genie, evil spirit).
Well I don't know if I explained it well. Take it with a big grain of salt because I'm studying japanese and I'm far from being at high level. I'm trying to study it every day and it is an hard journey, I can say it for sure if you are european and you use a total different alphabet.