John Carter's Barsoom is an 'end time' world, a relict ecology which has painstaking reassembled itself after a massive planetary extinction event and environmental collapse.

 It isn’t controversial then, that the Barsoomian civilization, or civilizations which exist in that age are also themselves the consolidated relics of prior civilizations.   Evidence of this abounds. 

 As one example, there is the fact that almost all Barsoomians of all races speak a common language. 

 Yet, there is ample evidence to suggest that other languages must have existed in the past.  This includes the exclusion of primitive White Ape speech.   The Great White Apes, notwithstanding their well deserved reputation for savagery, are also acknowledged to be intelligent.  Some of them wear clothes, they employ crude weapons, they gather socially, and clearly they are seen to converse with one another.  They are physically capable of speech, as we see in the brain transplant hybrids of Master Mind of Mars.   However, their speech or language, whatever it is, is foreign to the common tongue.

 There is a residue of other language words from the Black race.   More interestingly,  there are different written languages found in each city state.   The city state written languages are not easily intelligible to each other.    One has to assume that the oral language precedes a written language.  One also has to assume a written language has a greater portability across space and time.   So the only inference must be that each written language represented a distinct oral language at one time.

 Ethnic distinctions would suggest the evolution of different languages.   The humans and the green men represent entirely different evolutionary lines.  How is it that they speak the same language?  Surely it did not evolve together, and if one line adopted the others language, then surely it must have been capable of a language of its own, and if so, we can easily assume a proto-language or full language. 

 The other human races, red, black, white and yellow, the kangaroo men, even the kaldanes/rykors must have been isolated for a period of time to develop their different ethnic characters...  It only stands to reason that they developed languages as distinctive as their appearances.

 In short, for much of Martian history, the inevitable trend must have been towards linguistic diversity, much as we see on Earth. 

 So what happened to these languages?  Probably the same things that happen on Earth.  A dominant society emerges, its language becomes a trade speech of commerce and religion, and everyone picks up a little bit of the tongue in order to talk to visitors, strangers, traders etc.

 So where did the common language or trade speech originate?   It’s possible that, as on Earth, there were several trade speeches occupying history, with only the final one known to us.  The blacks of Omean in the Southern Hemisphere and Kamtol in the north may have had one of the first trade speeches. In both regions, they still retain the same or similar words of their original tongue.   The Okar appear to have made no contribution at all that we can find.

 The most likely candidate for Barsoom’s ultimate or final common language is that of the Orovars who, according to the histories, became a sailing and seafaring people, traveling the five oceans and dominating the planet.

 However, a trade speech does not necessarily mean that other languages become extinct.  Talking to strangers is one thing, talking to family is another.  Stable societies would have meant stable languages.

 It’s likely that the collapse of Barsoom’s ecology, and the destruction of stable societies meant the death of local languages.   When entire populations were on the move, when devastated societies united with other devastated societies for common defense or offense, when half the planet was refugees, then knowing the common trade language was far more important than speaking a local language.

 Under pressure of radical social change and upheaval, local languages literally disintegrated.  At times, speaking a local language could be a social death sentence, since it identified you as belonging to a weaker or subservient group. 

 The process of linguistic consolidation must have happened extremely swiftly, affecting all ethnic groups.   Only the First Born whose core habitats were ultimately stable retained any of their original language.   Even the Okar, who were able to escape the collapse by fleeing into their arctic refuge wound up adopting the common language and losing their own.  So the process must have been completed well before that time.