Fix copy conflict dialog translation

@wakeup @MorrisJobke @DeepDiver1975 OC-Dialogs uses plain html templates (which is a good thing). All translation is handled on the client side. Furthermore, apps should be self contained, so we use `'files'` as the source for our translations, not core. core might work for default buttons like *Cancel* or *Continue* ...

In general the code for the copy conflict dialog should move to the files app. But that is a task for a different PR.