Well, I
sort of disagree. As my handle implies, I'm named after a Numidian King (160-104 BC). I'm one of the "Kabyle people", a Berber ethnicity from Algeria, North Africa.
Although I was born in Algiers, we spoke Kabyle at home and most Kabyle people I know who were born elsewhere did, too.
If you go to cities like Tizi-Ouzou (one hour or less away) and Bejaïa, Kabyle is the lingua franca. That's the language people use to talk to each other. But you'd be understood if you spoke another Algerian language (Algerian dialects are a mish-mash of mostly Arabic, Kabyle, French, Turkish, Italian, and Spanish), or French.
So for me, I just go there and I can just speak Kabyle in a small zone containing a little more than 2 million people (there are many more in neighboring cities, and many, many more in other cities, but that's the "HQ" with the highest capita/km² speaking the language daily).
I use five languages daily living in Algiers (5 million people). Though I think you're partly right for the "retaining the language" part since Algiers didn't retain the "original" language: it mixed a bunch of them and then retained that.