I don't think that's quite what @henddraig said. She said it was in the slot (9pm BBC4, Saturdays) which is usually reserved for subtitled drama (very often from Scandinavian countries).
This is how it is currently being advertised in the BBC4 online schedule:
In Welsh and English with English subtitles.
If it's billed as "Hinterland", then it's going to be in a mixture of English and Welsh.
If it had been the all (or almost all) Welsh version, it would have been billed as "Y Gwyll". ).
But to be fair, I seem to remember that the 2nd series of Hinterland had a lot more Welsh in than the 1st series, so I hope this one does too.
BTW, Mali Harries had a R. Cymru programme recently about "Cardi Noir". (Well, a repeat of a programme from 2016 I think).
(She seems to have made quite a success of Gwyll/Hinterland semi-spinoff docus, but chwarae teg iddi. She's good actor, and deserves a bit of success / recognition).