I think it is quite difficult to get information about the welcoming of thongs. I have worn thongs and strings over a period of 10 years at beaches in the Netherlands, Belgium, France, Italy, Greece and even Turkey. Beside a few compliments and some rude remarks from drunks, I never got any other sign of being welcome. It is all public territory and only a law might prohibit it and that is very unlikely in Europe.
The USA is a totally different story. In the land of the free, some prude and/or religious minorities have the power to ban thongs and strings from public places. I wear my thongs at Sandy Hook, NJ. For years now and the rangers of the state park seem to be OK with that. This is my only experience in the USA. If you check out Aimoo's Thong Wearers Message Board, you will see comments about other places.