LibreTranslate is a free and open-source and self-hostable machine translation server. It also has a public instance designed for personal or infrequent use.
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Pro Free for light usage
The public instance is free for testing, personal, or infrequent usage, no need to pay for API usage or install for light use-cases. If you're using it heavily or on a commercial service though, you should really self-host.
Pro Uses Machine Learning
Unlike other open-source solutions, this one uses machine learning, not a dictionary, or rules-based translations. This makes it more comparable with Google Translate or DeepL.
Designed to be easy to self-host, you can control your data and usage, no need to send any content to anyone.
Pro Free and Open-Source
The code is released under the AGPL license.
Con Limited language support
As of 12/Apr/2021, it only supports around 13 languages.