How can a monkey learn sign language?

Answer 1

Lemurs do not have dexterous hands, so no, they cannot learn sign language.

Answer 2

Answer 1:

Repetition

Answer 2:

There is massive doubt as to whether they are actually learning sign language in the sense that most of us understand it. It should be noted that primates are not limited in language by their undeveloped vocal chords, but rather by their lesser brains.

In other words, while their hands are as dextrous as our's, they cannot sign with the proficiency of a normal human child, nor can they type even by the "hunt and peck" method in a realistic fashion.

They can master some symbols, and use certain ASL or keyboard symbols for that. Grammar and other rules are beyond them, and even their symbol limitation is staggeringly limited.

Movies showing an ASL proficient human freely "conversing" with an ape or chimpanzee are entirely fictional.