How to create a Germanic West Francia and a Latin East Francia?

The Romans need to lose the Gallic War and win the Germanic wars. The linguistic boundary basically follows the lines of Roman conquest.
 
The Romans need to lose the Gallic War and win the Germanic wars. The linguistic boundary basically follows the lines of Roman conquest.
Even then, the Gauls spoke a Celtic language, not Germanic, so they'd need to be invaded by Germanic-speaking people later. But not the mainland Germans, because they'd been incorporated into the Roman Empire, so maybe the Scandinavians?

This challenge is extremely difficult to pull off.
 
Even then, the Gauls spoke a Celtic language, not Germanic, so they'd need to be invaded by Germanic-speaking people later. But not the mainland Germans, because they'd been incorporated into the Roman Empire, so maybe the Scandinavians?

This challenge is extremely difficult to pull off.

Maybe the Roman conquest of Germania leads to a migration of people who then conquer Gaul?
 
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