Natlangs in Fantasy Worlds

I've always noticed that in fantasy worlds such as Tolkien's Middle Earth, English is often
used alongside conlangs.  This seems like it would be highly unlikely.

I recently had the idea of "devolving" English (or any other natlang) in a different direction
than OTL history.  Imagine making English actually evolve alongside conlangs in a conworld
to the point that it is actually identical to modern English, but with a different history.
(Sorry if this isn't clear--i'll make any clarifications necessary)  What do you think?  Would
this be possible?

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