OTL, only more so - there's a reason why the percentage of the population who were Welsh-speaking went from 90%+ in circa. 1800 to only about 20% or so now after all, and it wasn't because English was so much cooler and sexier.
But if you want a specific POD -
Brad yr Llyfrau Gleision. OTL the outrage in response to the report was such that the government never implemented the more extreme recommendations of the report, but as your POD you can have them double down instead. English-only education mandatory, the non-Comformist sunday schools shut down unless they switch to English, Welsh language publishers and newspapers forcibly closed, and so on.
As to why the English would be so brutal? For an earlier POD there's always
Rebecca. OTL, the authorities responded very sensibly to this movement which was a lot more violent than is often appreciated. What was in some places not far off a full scale peasant's revolt was treated more as an issue of public order and rioting, and it was dealt with pragmatically - a few high profile arrests and convictions, the law quietly changed to address the more reasonable concerns of the rioters, and the resulting decline in violence being used as an excuse to claim victory.
If the authorities instead went berserk - declaring martial law, executing people for treason, flooding the countryside with troops and so on, you could easily create a climate whereby it becomes mainstream establishment thought that a long-term restoration on law and order in Wales depends on properly extinguishing Welsh language and culture.