The game was marked as updated for me, but there's no changelog. Usually when this happens, it's that another port or language has been added, but as far as I can see they're still the same (Windows and Mac, English and German). Does anybody know what changed?

I wondered also. I guess nothing changed and it's another glitch. Happened a lot in the past with gog (marking games updated without any changes, there was one game that was "updated" every day for some days, forgot which one it was, but it wasn't jack keane).

Yep, it's a bonus update. The German walkthrough document has been added.