This is the only safe, legal way to get the official 1.39 version.

Step 1: Open Steam
Launch Steam and go to your Library.

Step 2: Find ETS2
Right-click on Euro Truck Simulator 2 and select Properties.

Step 3: Go to the Betas Tab
In the Properties window, click the BETAS tab on the left.

Step 4: Select Version 1.39
In the dropdown menu labeled "Beta participation," select:
temporary_1_39 - 1.39.x for legacy mods

Step 5: Download
Close the Properties window. Steam will automatically queue the download for version 1.39. Wait for it to finish.

Step 6: Launch & Block Updates (Optional)
Play the game. To prevent auto-updating back to 1.53+, keep the beta branch selected.

The Fix: This happens when you try to use modern mods on the 1.39 engine. When you download ETS2 1.39, you must also download mods that were released in late 2020. Never mix current-gen mods with legacy game engines.

I strongly advise against this. Pirated versions often contain malware, crash frequently, and lack mod support. However, if you ignore this warning: