Dach chi is just a regional variant of Dych chi. The meaning is the same, but it just represents the local pronunciation of Dych chi in some parts of north Wales. It is common enough that it has become accepted as an alternative written form in informal use in that area and it is also taught in that form on some local courses.

Dech chi is a common pronunciation in mid-Wales, but is not often seen in writing. Similarly Ych chi, Ŷch chi in parts of south Wales. There is a brief introduction to the four or five main dialect areas in the notes for the section 'Dialects', together with links to other sources of information.

Most locally-based Welsh courses for adults will introduce local dialect variations at an early stage.

If you go on to learn Welsh at a more advanced level you will cover more of the common dialect variations heard in the media. You will also cover the more formal registers of Welsh commonly used in the written language and in more formal speech.

This was supposed to be 'dych chi' as the best answer with 'dach chi' and others as alternatives. However the best answer was set to 'dych chi' + 'dach chi' + 'wyt ti' so the system used the one that comes first in the alphabet.

Apologies for the confusion, that's fixed now.