Beyond the official changelog, reverse-engineering by the modding community revealed two critical changes in 1.6.1:
Converts from 1.5.2 to 1.6.1 are . However, you cannot revert a save to 1.5.2 after loading it in 1.6.1. Mods that rely on old cargo definitions (pre-France) may fail. Euro Truck Simulator 2 Patch 1.5 2 To 1.6.1
The core of this transition was the 1.6.0 release, which focused on connectivity and multimedia: Beyond the official changelog