Hello! I live in Japan now, but am originally from Europe (half UK, half France) so I know most of the vegetables you mention. Salsify -- or salsifis in French -- is still not as well-known as carrots, but it is a staple of French cuisine and regularly features in restaurants: anybody half-way serious about traditional food will know it, I would say.

As for dandelion -- pissenlit in French, which means ... to "pee in bed," inreference to its duiretic qualities -- you can buy it it high-end and slightly hipsterish greengrocers quite easily. Same in restaurants: it's regularly used in salads all while it's in season. I've got one cultivar I'll be sowing later on this year (for both leaf and root harvests): it's called Amélioré à coeur plein "improved, with full heart".

As for the last two, I know them a little less, but I would still say they are relatively common somewhere like France, where they both grow naturally along the coast of Brittany.