[Davros]

A little explanation from the book that DM is using :

" Deverry is a P-Celtic language - although closely related to Welsh, Cornish and Breton, it is by no means identical to any of these actula languages, and should never be taken as such.

DD is the voiced "th" as in thin or breathe - if your using the harsher form (ie The or breath) then you use TH

Y is never a consonant

DW, GW, and TW are single sounds as in Gwendolyn or twit"

So in Deverry speak, it comes out as "Gwerith". How close this is to actual Welsh remains to be discovered. I imagine it won't be too far different.