Oes Visa gennych chi? - Formal.
Oes Visa gyda ti? - Informal.
Mae tegan newydd gyda fi.
"You are Welsh" = Rwyt ti'n Gymreig
I can't say about other Celtic languages, but Welsh, I would say, is getting stronger. Since the 80s the Welsh Language is being learned at schools. Don't forget that at one time English law prevented Welsh being spoken at schools and if you spoke Welsh you had to wear a small wooden plank and the one got to wear it last of the school day got beaten by a cane. Some jobs are advertised in Welsh to increase the Welsh-speaking work force.
'Our Friends' in Welsh is 'Ffrindiau Ni' :)
To say "your house" in Welsh, you would say "eich tŷ".
Cymraeg
No in Welsh is dim.
"y diwedd" is how you say it in Welsh language.
In Welsh, you would say "Rwy'n cytuno" to say "I agree."
Draig Gymreig
You can say "it's nice" in Welsh as "mae'n braf."
Great > [Welsh] mawr