What explains why Paris France is much milder then Fargo North Dakota even though they have the same latitude?

The latitude of a location only explains the amount of solar energy that reaches the location. It does not explain other important climate factors, such as ocean currents and wind patterns.

Paris is located in the mid-latitudes, where the climate is generally temperate. The North Atlantic Drift, an extension of the Gulf Stream, brings warm water from the Gulf of Mexico to the western coast of Europe, and this helps to keep the climate in Paris mild.

Fargo is located in the northern Great Plains, where the climate is generally continental. The cold Arctic air that comes down from Canada often reaches Fargo, and this can cause the temperatures to drop significantly.