And Keenser *can* speak. He only says one word in the film (when Scotty says "Unless you know of some other half-starved Starfleet officer" and Keenser pipes up "Me!"), but it is there.
Before Keenser appears for the first time - right after Kirk and Spock Prime first enter the Delta Vega facility - we hear some things that might be Keenser speaking in his native language. It's right before we see him appear at the end of the corridor and start walking towards Kirk and Spock.