Akuma is known as Gouki in the Japanese versions of Street Fighter.
“Akuma” is the name used in international releases, while the Japanese name is Gouki. The distinction began with the character's debut in Super Street Fighter II Turbo, where he appeared as a hidden fighter and an alternative challenge to M. Bison. Gouki is written in Japanese as 豪鬼, a name associated with overwhelming or fierce demonic power.
The localization difference is more than a pronunciation change: “Akuma” means “demon” in Japanese and was selected for overseas releases to emphasize the character's terrifying image. Both names refer to the same fighter, the younger brother of Gouken and a master of the Satsui no Hadou, the destructive force that shapes his story.
Oni is not his ordinary Japanese name; it is the name of a separate, more extreme form introduced in Super Street Fighter IV: Arcade Edition. Gouken is his brother's name, while Gen is another Street Fighter character entirely. Those distinctions make Gouki the canonical answer for the Japanese-version question.