If you specifically need version 1.39 for a certain mod, Steam allows you to "downgrade" easily. Just right-click the game in your library, go to Properties > Betas , and select "temporary_1_39" from the dropdown menu.
To get the most out of version 1.39, combining the base game with its wide array of Downloadable Content (DLC) is essential. As of this version, the following expansions were highly sought after: Popular Expansions for 1.39 Euro Truck Simulator 2 1.39 All Dlc Download
Then the game launched.