How can Mouse's coven understand Nessa and Marie, and vice versa?

In Craig Sheaffer’s Wisdom’s Grave trilogy (an interlude between two Harmony Black novels Right to the Kill and Black Tie Required), at one point the two main characters, Nessa Fieri (Roth) and Marie Reinhart find themselves in the world of The Revanche Cycle — and somehow have no trouble understanding anyone’s speech, and can apparently even read the written language. Later, Mouse’s coven lands back in Maria and Nessa’s home world (our world?), and again, they can understand and read English.

Did the author ever explain how this happens? I read these books over the past few weeks, and if so, I missed it completely. Or it is just "plot translation?"