"Dych chi'n" I am familiar with: where has "dach chi'n" come from?
It is the NW Wales pronunciation of dych chi, and appears in informal writing there quite often. It is also taught in some regional Welsh for Adults courses, which is why we include it in Duo. Unless you live in the area, just be aware of it.
Thanks. I live in southern England, actually, and never come across Welsh, except here.
I forgot to mention that it is mentioned in the notes for the 'Dialects' section earlier in the course.
If you keep an eye on http://www.bbc.co.uk/cymrufyw you will see it there from time to time, along with other dialect and register variations.