Depends on what you project the die off to be, but realistically if you do a real TEOTWAWKI event, say 90% dead, with termination of international and national media (and possibly severe attenuation of literacy at all in a lot of places), 150 years isn't too long to get you to a US map now occupied by numerous mutually unintelligible languages all based on 20th century English.
It's a profound punctuated equillibrium event -- no more equillibrium, 150 years of free falling scattered tribes and statelets. Put that in a beaker, shake and "everybody mostly speaks English" coming out the other end is unlikely.
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