Am I allowed to be a little unrealistic for middle ages?

[RealJasonBourne]

The guy needs to know sign language but it wasn't widely used until maybe 1700. If I have a guy in the 1500's (medieval ages)
who knows sign language and he communicates with someone else who happens to know it, is that crossing the line or will they let that slide? (audience/anyone).

does it have to be that realistic or can story be something that doesn't follow facts?

What are my best solutions here? scrap it? or go for it because its tv and on a subject no one really knows about?

edit: sry for bad sentence function lol

 

[RealJasonBourne]

In regards to what APE said: the way the deaf were generally treated can be great setting for some drama where the love of family and views from society can clash in a terrible (and terribly emotional) way...
Or the deaf person is a cast out run away. (Which can open the door to a hunchback of the notre dame/monster of Frankenstein like drama.)

Thanks!