I noticed on the 1.03 patch noticed that Japanese voice and text was added. I downloaded the japanese voice from GOG Galaxy went ingame and the only language option i found was the subtitle one.
How do i turn the japanese voice over?

Anyone had it figured out?

I'm outside right now and can't check exactly where I did it, but I'm sure I set the language in Galaxy and not in the game. (And yes: I also switched from English to Japanese after already having played.)

I gotta change the Galaxy client too in order the game to turn to japanese voice? What about the interface of the game? Does it stay in english?

Sorry for the late reply. Haven't been at my computer the last two days.

You do not have to change the interface language of the Galaxy Client, but inside the Galaxy Client select Witcher 3 and from the "more" menu to the right side of the play button pick "configure" and from the drop down menu on the config page pick Japanese. After I did that my game ran in Japanese the next time I launched it as far as I can remember.