DeepL does pretty good quality translations, even better than Google Translate sometimes, but it is also proprietary software-as-a-service operated by a single for-profit company. One of the features of the paid version (which is explicitly not the case with the free version) is "All texts are deleted immediately after translation so they can never be accessed by third parties."

See other links in this thread for free/libre open source alternatives which can be run locally and offline. I'd say that for common european languages, at least, Argos Translate is not quite as good as DeepL or Google but it is more than acceptable (and even the best proprietary things are far from perfect).