"Where used you to work?"
Translation:Ble o't ti'n arfer gweithio?
What would you have written?
Two options that come to my mind are "Where did you used to work?" and "Where did you use to work?" but I'm sure there are people who will find fault with both of those!
I'm not sure whether there is a good, standard yet natural way to ask questions with "used to".
This is a link to an older deleted English translation. Mizinamo's suggestions are the ones already marked as best translations.
"Where used you to work?" is what a dignified, well-educated, older person might ask. It is maybe a little more "upper-class", and I like it!
"Where did you used to work?" is a good, common, every-day, less-formal question. The tone here is slightly rougher and less "snobby".
In my English-speaking country, both these questions are perfectly natural.