Apertium, a free open alternative to Google Translate, is currently under development (https://apertium.org).
However, there is some disagreement in the development about whether to use certain third party services which may affect privacy:
https://github.com/apertium/apertium-html-tools/issues/288
I'm not a developer, so I don't know what the technical trade-offs are here with viability vs privacy.
However, if you are a developer interested in the project, please do make your opinion known in the thread above.