。
She translated the letter from Japanese into French.
Literal
She [topic-は] that letter [object-を] Japanese [from-から] French [to-に] translated.
~から~に ('from X to Y') is the canonical pair for source and destination — から marks origin, に the endpoint. Used here for languages, the same structure appears for spatial movement (東京から大阪に, 'from Tokyo to Osaka'), times (朝から晩まで, 'from morning to night'), and transformations (固体から液体に, 'from solid to liquid'). 翻訳する ('translate') is built on 翻 ('flip, turn over') + 訳 ('rendering, version') — translation as 'flipping' a text into another guise.