When you are writing for multi language audiences, humor 

a. Should be used often because it makes your audience feel welcome on a personal level.

b. Should rarely, if ever, be used because humor is one of the most difficult elements of communication to encode or decode in a second language.

c. Should never be used because movies and other entertainment products rarely cross over national boundaries.

d. Should be used at least once per letter to show that you appreciate your audience as human beings.