Not so! You, and all the others who say this isn't possible are being very naive about this. Up to about puberty human beings are programmed to learn languages easily and naturally, and can learn several to a high standard. After puberty it becomes more difficult; you're more likely to have a definite accent, but a new language can be learnt. There are millions of people OTL whose regular daily language is not their mother tongue. It is even possible through lack of use to effectively forget your native tongue.
So language change is possible. Furthermore it can be forced. Language is the principal carrier of civilisation, and this can lead to conflict. Since the beginning of C19th the history of romantic nationalism, at any rate in Europe, has been one of language conflicy, with 'nationalists' attempting to reinvigorate or restore their local languages, and 'imperialists' trying to discourage them (or worse) in favour of sngle 'civilised' tongues. Think how successful the French state has been in stamping out local languages in France since the revolution. Think of Franco's and Mussolini's language policies. Look up 'vergogna' and 'Welsh Not'.
So it would be quite possible to start similar 'language suppression' policies much earlier as @ Intransigent Southerner has suggested - remember eg the Ancient Greeks thought that speaking Greek was the mark of a civiliSed person; those who didn't, merely babbled 'ba-ba-ba' - they were barbarians. The question is why, Language is one of the main ways to worship the gods. To enforce speaking a particular language while not worrying about how and who people worship seems contradictory.
Perhaps a Secular mindset develops much earlier in this world. Perhaps great rulers arise who think they are the epitome of civilisation - therefore everyone must speak like them. Perhaps poets and storytellers develop great epics which, they maintain, cannot be translated, the meaning is in the actual words themselves. Marketplaces develope where for trading to be fair and equal everyone must speak the common language. those heard speaking something different are punished by the market authorities. And of course as great empires develope, a single language is more administratively efficient, and the ruler wants everyone to speak a single language so he can more easily tell if anyone is plotting against him...
Education would have to be developed very much earlier. Children would be encouraged to report relatives, even their parents if they heard them using an old, forbidden language... Hm, it could be done