Most of those browser add-ons still use Google Translate, unfortunately. And when I say ‘most’, I mean ‘good luck finding one that doesn’t.’ Instant Translate, which you mentioned, appears to be closed-source. I would love to see a browser extension that uses Apertium.
This is what I’ve been using. And it seems to work alright. The reviews report a lot of odd behavior, and the possibility that it does not respect privacy. Personally, I have not noticed any of these issues. However, you could chalk that up to the fact that I’m not on Windows, and have fairly heavy privacy controls.
While I fully support the idea of auto-translating on Diaspora*, it really seems like a moonshot kind of effort. I want it eventually, but adding translation to all the other things pods are doing would be computationally expensive.