"Do you want a Welsh school?"
Translation:Dych chi eisiau ysgol Gymraeg?
Because eisiau (isio, etc.) doesn't take yn. (Its grammar is a bit odd since it's really a noun rather than a verb, as I understand it.)
Also, "rwyt" with its r- is used in positive statements but not, I believe, in questions (or in negative statements) -- there, you need "wyt ti?". (And negative would be "dwyt ti ddim", among other forms.)