It looks like the decline from 2001-2010 would have continued if things had been left to evolve naturally.My bad, I was reviewing my info when you posted.
However, Welsh Gov does an annual pop survey... Bit of a difference, eh?
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I'd like to see that split out by age group, it's likely that most of the increase in welsh speaking is by children and young people, who had it forced upon them at school.
The whole thing is a very weird social experiment, in reversing natural evolution. I don't think there's any logical reason for this sort of indoctrination, it's more about politics than education.
There's no problem with learning Welsh, but these school hours are taken up with learning something more useful outside of Wales, perhaps French, German, Science - things that will further their prospects.
Also other subjects are being taught IN welsh, so their learning in ALL other subjects is likely to be less effective.