A few comments on this and that.
One aspect of EU membership which hasn't been mentioned is the existence of
the Minority Languages money. Financial support is given to projects which
promote minority languages throughout the EU. Not a reason for staying in,
but not to be forgotten.
I'm mildly curious as to how (or whether) the Welsh-language supporters
propose to prevent non-Welsh-speaking Welsh people from moving within Wales.
There are plenty of them.
The maltreatment of children for speaking languages other than that of the
dominant culture is not a uniquely English trait. Breton speaking children
in France were similarly treated, Basque speakers in Spain likewise I
believe. I suspect every country could provide another example.
I'm not denying that it happened here, but it was part of a much wider
consolidation of power as nation states emerged.
I have a recollection of being taught that immigrants to the US were forced
to learn English in order to gain naturalisation, but maybe I'm wrong there.
All the best