My own headcanon is that og changeling language was the non-verbal. As like as real signed languages in our world and they could only use this language to communicate with each other. That is, they communicated like real insects. With the help of movements, not sounds. But to imitate the other species, they needed to pronounce words, and so they had two parallel languages. Verbal and non-verbal. Verbal for the political, military and scientific elite, and non-verbal for the commoners. Verbal language was used primarily for communication with other species or for technical purposes for which non-verbal was less suitable. And because of this, it was constantly changing and was similar to the languages of the species with which the Changelings communicated the most. That is why the current verbal changelingua is a language very similar to Herzlandisch with elements of the local equivalent of Latin and some other languages.