Translation:Have you gone to the party on Friday?
A strange sentence in both languages! But then finding these oddities is why they are beta-testing the course.
Dych chi wedi mynd i'r parti dydd Gwener? translates as 'Have you been to the Friday party?
...y parti dydd Gwener is definitely the Friday party just as parti Sioned would be 'Sioned's party', or y parti diflas would be 'the boring party'.
...mynd i'r parti ddydd Gwener, with the mutation of dydd to ddydd translates as '...going to the party on Friday' - the mutation tells us that ddydd Gwener is referring back to the verb dych chi'n mynd.
dych chi wedi mynd...? is 'have you gone...?', not 'did you go...?'.
'Have you gone to the party on Friday?' is plain bad English.
Back to the plot, and a pair of sentences that are equivalent and which work in both languages:
Aethoch chi i'r parti ddydd Gwener? - 'Did you go to the party on Friday?'