Not historian, but I know of several cases: Most language changes are actually coming from a will to come back to a past; therefore we have Israel and Hebrew, Ireland and Irish. They changed their language and began speaking a language that was nigh extinct for political reasons. America changed its English for political reasons too. But it's still English. Then you also have countries changing their alphabet - say Turkey and Vietnam to go to Latin. And of course most Europe had Latin has an official language before switching to an other one. And France was not speaking French for a very long time, so you could count that at "peacefully" changing the language (Gascon, Breton etc), even though it wasn't that peaceful. But I can't think of a case where a country would have just changed its language if it were long established in an independent country.

Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again.

I don't know of any dramatic examples of the kind that you're looking for, but there have been some cases of nations adopting a second official language because of its prevalence in the region, or its importance to trade.

This happened as recently as 2012, with Latvia making Russian an official state language. It's also why Chinese is one of the languages of Mongolia.