"Are you eating sausages?"
Translation:Dych chi'n bwyta selsig?
Dialect mostly. "Dach" is more typical of northern dialects and "dych" of southern.
when do you put 'dych chi' at the beginning and when at the end, like in 'sut dych chi'?
Put Dych chi...? at the beginning of a yes-no question such as "Are you eating sausages?"
Put the question word such as pam (why), sut (how), ble (where), pwy (who), beth (what) first in a WH question -- a question that is not answered by "yes" or "no" but that has a special question word (WH word) in it. (So called because most question words in English start with the letters wh-.)