this might not be the first language known to 'humans'. But, no matter what creature you look at, all of them have a language they speak (usually non-verbal).
-Insects are probably, and most-likely, the longest-living species on this planet. Each speaks in its own way. example: a bee 'dances' in its hive to tell other bees about intruders, food or it could carry a message from the queen.
so, if you think about it, i guess this could be the first language ever used...