Prescriptivism, of course, is all about power: who gets to talk how where, and who gets to say how they should talk. As good libertarians, our first reflex might be to say that this is all unnecessary: let everyone decide for themselves! That has two different problems. The first is that, when people decide for themselves, what they end up with is in fact a set of implicit rules for what's appropriate in which circumstances, and if you want to make life easier for visitors from other cultures, the least you can do is make those rules explicit somewhere. The other is that, in the event of any clashes, it's the more powerful individual that gets to decide, which is a particular problem in the case of public services. You want a driver's license, and you only speak English? Sorry, our local transport officials aren't really comfortable with English, so you'd better brush up on your Russian.
The latter example may sound like fantasy to American or English readers (not so much to the Irish or Welsh), but it's rather close to reality in a lot of the world. If you understand Arabic, have a look at this video of Moncef Marzouki, one of the two current presidential candidates in Tunisia, having a go at his Tunisian interviewer for using too many French words: "Respect the Arabic language! Plutôt, what does plutôt mean? You say plutôt, what's that? My sister in Douz won't understand plutôt. [...] [Interviewer: It's a chance for her to learn...] No, she needn't learn - you learn the language of Tunisians!"
It's populism, of course - but, like a lot of populism, it makes a good point. Why the heck should the average citizen have to speak a foreign language to deal with officials and other elites in his/her own country? (Especially in one as close to monolingual as Tunisia?) In such a situation, if the populace doesn't prescriptively impose their language preferences through concerted action, the bureaucracy will simply impose their own in one-to-one interactions.