In Bone Crossed, p272 the oakman speaks to Mercy in 'English-accented Welsh' and she doesn't know how he knows that she speaks Welsh.
In Iron Kissed p54 Samuel and Tim are speaking in Welsh (and Tolkein's Elven) and Mercy is thinking 'I may have grown up in a werewolf pack led by a Welshman, but English was the common language and neither the Marrok nor Samuel used Welsh often enough to give me an ear for it.'
There is essentially no time between Iron Kissed and Bone Crossed, so when did Mercy learn Welsh?
Maybe it was a typo. Mercy knows German, so maybe the oakman was supposed to be speaking in German.