^ ^ ^ Tots agree.
It seems extremely unrealistic for an entire species to have developed one language. However, by the 23rd century, and unified space travel, humans had unified english as the official Terran language, so it's not completely unrealistic for the Klingons to have undergone the same sort of unification. Nevertheless, Klingons don't exactly center their culture on such unification or academics, so a unification process may have occurred slowly, for generations, and may have still been occurring at the time of TOS. But since the TNG people used a common reference source, the language spoken in TNG was uniform. In-universe, this means the Klingons developed a unification in the century between the two, not implausible at all. The dialect that finally became custom in-universe in TNG may have differed greatly from TOS, explaining the difference in "official" languages between the two.