While this does not exactly answer your question, one way to keep a view restricted in “crunch times” is using hide using topics incl. language tags as discussed here. Then, if desired or during our free time, we can visit those tags and posts easily.
Given that most of the modern browsers do provide translation facility on mobile and desktop natively, I wonder if having something within forum as an extension would add much - but thanks for revisiting this, would be interesting to see if things have changed since the last time