Within the marvel universe I don't know.
It's called Illeism
Illeism /ˈɪli.ɪzəm/ (from Latin ille meaning "he, that") is the act of referring to oneself in the third person instead of first person.
Illeism is sometimes used in literature as a stylistic device. In real life usage, illeism can reflect a number of different stylistic intentions or involuntary circumstances.
As far as I know its' not revealed in canon, but we can surmise:
Mantis is of Asian ancestry (obviously):
The enigmatic Mantis traces her roots back to the family of Vietnamese crimelord Monsieur Khruul who, disapproving of his sister Lua's marriage to German mercenary Gustav Brandt, hunted the couple across Indochina¹.
Created by Steve Englehart who also create the other martial arts expert Shang Chi indicating that he must have at least a cursory acquaintance with Asian customs.
Look at this question it's not unheard of as a speech mechanism.
Aside: Mantis will apparently feature in Guardians of the Galaxy Vol. 2.
In October 2015, del Toro was confirmed to return and Pom Klementieff was cast as Mantis.