I found tutorials online, which show how to change the UI from english to japanese (for the steam version). In order to do so the japanese data files must be part of the distribution.

As I want to play this game in japanese (UI included) I need to know if the GOG version supports it..

Could someone who has the game installed please check if there is a [JP] folder (without the brackets) inside the install directory and whether it contains any files. it should contain the following folders:

JP-
----Accessory
----ActionScript
----Font
----GameText
----Item
----Script
----Sound
----UI
----Video

Thanks!

I'm gonna answer my own question here (as I have bought the game now)

yes there is a JP folder and the game can be changed to Japanese (with some minor flaws)