I think it depends on your instance. In mine we are offered to use LibreTranslate.
I didn't know DeepL, but it's a good looking option. Nevertheless, it's not free software, so you can find some day they deleted the free plan, or started forcing customers to give them any personal data they want.
On the other hand, you can self-host your own LibreTranslate instance (it doesn't need as much resources as you could think for personal usage).