I'm having just a doubt about the second line of the "if"

  $request->attributes->set('_vary_by_language', true);

It is later used by the ResponseListener to send a Vary header.. so i guess we want to send this header regardless of whether the client send its preferences or not (or that coud generate big cache problems)

(sorry, did not think about that when I wrote the example)

Indeed. Nice catch @smnandre

Thanks, I added a test case for that @smnandre

Isn't something we need to merge in 5.4?

@fabpot yes, good catch.

Thank you @jkobus.

Thanks, happy new year for you all 🎊