If your characters are traveling to the 11-1400s, then they can understand them a little better than if they were traveling to the 5 or 600s. Either way, they'd still have difficulty in conversation and would need a dictionary to communicate effectively.
For the sake of accuracy, your character could probably stumble around some of the words and get the 'gist' of what the French are saying, but maybe not understand explicitly.
When it comes to pulling the reader along though, it may come down to a stylistic decision. Case-in-point, the movie Timeline. In the story, English-speaking archaeologists travel back in time to 1357 Castleguard, France where there's a huge war between the French and the English. For the movie, they used modern French, so it was easy to pick up what was being said without reading the subtitles. I'm not sure how it was handled in the book since I never read it, but the author is usually picky about his stories being accurate with the modern data of that time.