On this page of the Assassin's Creed Valhalla Siege of Paris guide, you will find a detailed walkthrough of the To Francia main story quest.
This mission will take you to France. You don't have to worry that this is a point of no return though, you can move to a different map at any time.
Speak to Pierre to Ravel to Francia
After completing the Strangers Bearing Gifts quest, you will be able to travel to France. Wait a while for your character to sober up, though. For several seconds, your character can feel the effects of an all-night feast.
During the conversation, you can confirm that you are ready to move out. You will get on a ship sailing to the foreign country. You will immediately be able to see that the new lands are rather bleak - the sun isn't shining like it did in England or Ireland.
Once the game loads up the new map, you will be given another mission - Warlord of Melun.