Improves the implementation of non-verbal languages

Cleans out the nonverbal language check from say_understands(), which
now is responsible for language only.
Nonverbal languages have a chance of causing an emote when used.
Adds sign languages, which have no verbal component.

(cherry picked from commit ac0bf7b8539f3d3dd684cfc8bd0a0eeb4c79587c)

Conflicts:
	code/modules/mob/living/say.dm