This question assumes the correctness of the Shita that it is forbidden to use Lashon Koidesh for mundane language today. Rather I'm asking if this will or will not be the case in the restored Kingdom of Israel.
I don't know if the Satmarer Rebbe specifically wrote on this or not, in Vayoel Moishe or some other Seifer.
But when the King-Moishiach comes and restores the Kingdom of Israel, will the Lashon Koidesh be restored as the language of day-to-day affairs, or will Yiddish remain?
Some thoughts and possibilities:
The assumption being that it would be no longer required to abstain from speaking Lashon Koidesh in day-to-day affairs if in the time of Moishiach, Klal Yisroel would no longer say anything forbidden. Thereby Lashon Koidesh could be restored as a general language, and Yiddish et al. could be discarded.
The assumption being that it is not enough to merely abstain from speaking forbidden things when speaking Lashon Koidesh, and instead that it is forbidden to speak of any mundane thing at all when speaking Lashon Koidesh. Thereby with Lashon Koidesh being reserved only for holy affairs, if in the time of Moishiach, Klal Yisroel will no longer speak of day-to-day mundanities and only of Toirah, then Lashon Koidesh could be restored as a general language. If not then Yiddish et al. would remain in such a function.