I've used DeepL through the Translate You app by Bnyro. What I liked the most about this app was, as the name indicates, the Material You design. Unfortunately I don't use it very often because DeepL has a limit of queries you can make per month(?) in the free tier, and Translate You doesn't give you a button to send your translation query, updating what you type in real time just like the official DeepL website, making you waste queries faster. Other than that the DeepL service has been great.

In the future, please refer to other subreddits for privacy minded app recommendations, like r/PrivacyGuides or r/PrivSec_Dev as this question is not relevant to GrapheneOS. Members of the GrapheneOS community also talk about app recommendations in our official Matrix off topic room, you can get there through the link in the automod reply.