My father translated a German letter into Japanese.
Literal
Father [topic-は] German letter [object-を] Japanese [into-に] translated.
訳す (やくす) is the native Japanese verb for 'translate' — more compact than the suru-verb 翻訳する. The pattern is Xを Yに訳す: 'translate X into Y.' ドイツ語 follows the [country]+語 naming convention.