While you use the Translate app, your voice interactions and input text are sent to Apple to process your requests. Your requests are associated with a random, rotating identifier, not your Apple ID, email address, or other data Apple may have from your use of other Apple services. By default, Apple stores transcripts of your voice interactions and your input text for two years and may review a subset of these transcripts. The small subset of requests that have been reviewed may be kept beyond two years, for ongoing improvement of Translate and other natural language processing features such as Siri.
You can help to improve Translate’s voice translation by allowing Apple to store and review your audio interactions by opting in to Improve Siri & Dictation. To do so, go to Settings > Privacy > Analytics & Improvements and tap to turn on Improve Siri & Dictation.