Free and Open Source Machine Translation API. Self-hosted, offline capable and easy to setup. Run your own API server in just a few minutes.

A Free and Open Source Translation Service. Can translate text and text files(Including Word documents, epubs, and HTML files).
It also includes a simple and well-documented API, Albeit paid.
It can also be self-hosted, giving you the option to run your own API server.

Github link: https://github.com/LibreTranslate/LibreTranslate