For postdocs @ ETH:
If this is not said specifically in the job text, you can safely assume that no knowledge of German will be required, although it may or may not count as a plus depending on the PI. There will almost certainly be no statement in your contract that you will need to learn German within a time frame, as postdocs are not "career" positions in the first place (and neither are they particularly teaching oriented, which is the only place where ETH potentially cares about you speaking German).
For professors @ ETH:
Unlike most other European universities, ETH is actually fairly international. I would be surprised if all of the faculty from around the world teaching there speaks German, although many that I know do (as you say, learning the local language is generally helpful if you stay in a foreign country for a longer period of time). German language skills will almost certainly not be required at time of appointment, but it may become part of your contract that you are obliged to be able to teach in German within a given number of years. This requirement may be negotiable, with the main concern from ETH's side being that this would limit what courses you will be able to teach. However, honestly, I would not worry about this too much at this stage.
For other European universities:
As usual, customs vary a lot across other European universities. For instance, I have seen announcements for German universities where PhD students were required to be able to speak German. That being said, the rules from above are good guesses in the absence of contradicting further information - don't worry about it for postdocs (unless the announcement specifically says that you need to speak the local language), and don't worry too much for faculty positions, but be prepared that you may be obliged to learn the local language.
