Never even seen one on a GA aircraft
3/4 of my clubs fleet is IFR capable and has mode S fitted since, I guess, six months to a year or so. We have deferred upgrading the VFR-only aircraft for now.
You're welcome to take a look but since a mode S transponder looks virtually identical to a mode C transponder, there's not much to see.
We had one of these safety awareness briefings at the club just this week done by the LVLN (the local variant of NATS). They also invited a few controllers and they've confirmed that technically all the radar systems have been upgraded to mode S. Mode S is already visible in the technical rooms (where the computers are located) and there's still a few minor issues to sort out with regards to training, modification of procedures or something, but by april/may or so, mode S readouts will be visible on the controllers radar screens as well.
One remark they made was that you can modify the flight ID from the panel. So far they've seen the strangest flight IDs, "JESUS" being the example they mentioned. Especially if you fly a rental aircraft, check the ID before take-off. It should be set to your callsign obviously. If not, they can track you by the 24-bit ICAO ID (which you cannot modify) and give you a verbal lashing I guess.
Need to speak the local language though
Not fluently but it helps a lot to have a crib sheet with the most commonly used aeronautical terms, particularly for whatever is said in the circuit. The French for instance, reason that since they're French, flying a French-built aircraft into a French grass strip, have no reason to speak English. Crib sheets for various languages can be found on a number of flying club websites.
En-route and on the larger airports, English is no problem.