EU Licence Plates
The EU countries now have new registration plates incorporating the country code and the EU flag on the left. This voids the need for the former oval sticker, enforced by the Vienna Convention on Road Traffic. Quoting from the "Council Regulation (EC) No 2411/98 of 3 November 1998 on the recognition in intra-Community traffic of the distinguishing sign of the Member State in which motor vehicles and their trailers are registered":
Member States requiring vehicles registered in another Member State to display a distinguishing registration sign when they are being driven on their territory shall recognise the distinguishing sign of the Member State of registration displayed on the extreme left of the registration plate in accordance with the Annex to this Regulation as being equivalent to any other distinguishing sign that they recognise for the purpose of identifying the State in which the vehicle is registered.
See for example Italy and France:
For more information, here is a wikipedia article.
Swiss Licence Plates
Cars with a Swiss licence plate are obliged by the Vienna Convention on Road Traffic to place a CH sticker on their car, when travelling inside the EU zone.
Liechtenstein Licence Plate
The Liechtenstein licence plate does indeed contain the country indicator. This is
FL which stands for
I could not find an authoritative reference on the topic. In my opinion however, the
FL marking can easily be confused with the equivalent of a region/city coding (such as those found on German plates). To be safe, I would suggest you do in fact stick a country indicator oval on your car, to avoid problems with over-zealous police officers. I can't imagine how hard it would be to try and convince foreign police that
FL is indeed a country indicator, by quoting them the Vienna Convention.