According to Toyota, Yaris is a combination of the name of a goddess in Greek mythology, Charis, and the German expression of agreement, "ya," because "we think this new name best symbolizes the car's broad appeal in styling and is representative of Toyota's next generation of global cars."
^^^Hope that helps. I know there are more sources but that's from one car review of the Yaris. You probably already know this car is sold as the Vitz in Japan and some other places to my limited knowledge.
Anyway the Yaris is a combination of names or meanings and no its not Japanese.