What if the bats incorporate supersonic chirps into their language? Then it would be impossible for mere mortals to learn!

Unless they made like, clicking your tongue or something a substitute for said chirps.

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Response: This Unit used to be able to do a reverse whistle that may have been a possible substitute.
Informative: Now it finds such things irrelevant with mechanical speakers.

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Informative: This unit can NOT whistle very well but can make popping noises by opening and closing the mouth very well in fact when the unit pronounces words that end with a P sound there is slight pop to it.

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They could even write in dead letters.

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Usually I assume that they use regular language... But most creatures just can not hear them because ultrasound. Also in my headcanon batponies are incapable of producing normal sounds, so to speak with anyone who is not batpony, Changeling Queen or alicorn they need special talismans.

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Supersonic language? Wouldn't that be devastating to everyone nearby due to the insane noise that occurs whenever the sound barrier is broken?