From the changelog thread on GOG's subforum for this game, it appears these were the changes since 1.2.1.3:
- 1.2.2.0 from 2017-11-16 - "Japanese localization now available! Please select your language option of choice when booting up the game."
- 1.2.3.0 from 2018-02-09 - "Added cloud saves, Added overlay support, Added support for Simplified Chinese"
GOGdb entry appears to indicate there were no further updates, so this should be the state of the game as of the time of making this comment.
The cloud save and overlay support would be GOG Galaxy-specific so, unless there were more changes not mentioned in the changelog, you're losing extra language support by buying here. I wonder if there's going to be further development of the game, and if the itch version is abandoned then? Buyer beware in any case, I suppose.