The Welsh here is the emphatic form stating that it was 'they' that were to blame. I took the English to be the non-emphatic form. If it were emphatic in English, I would say "It is they who are to blame".
while i agree that's technically correct, i feel it would be very peculiar to hear someone say "it is they who are to blame" in natural conversation. i would just put emphasis on the word "they" or say "blame THEM"