I know dozens and dozens of folk(ish) songs, including most of those listed by Les, and can sing them by heart, but if asked to lead a group of non-folkies in song -this is in England - I'd be scraping around to find many that they knew more than a few lines of. Apart from hymns, carols, God Save the Queen, and Happy Birthday, there's not much of a central core. I supppose there are:
Ilkeley Moor
Clementine
Molly Malone
You are my Sunshine
Auld Lang Syne (but only one verse)
You'll Never Walk Alone (and this only because of its use in football)
When the Saints (ditto)
Drunken Sailor
Tipperary
Pack up Your Troubles
Daisy Daisy
... but I'm not sure whether young people would all know even those now. And it's sad to note how few of the above are even English or traditional, let alone both. Nor do I think (to answer Grishka's question above) that young people sing pop songs - or indeed anything else except perhaps a football anthem - in pubs, at least not in my hearing.

There are a lot more songs that other people will recognise and even request, and they may join in with choruses and refrain lines, but in most cases they won't be able to sing more than about half a verse, and would need a competent person (teacher, scout leader,folkie) to lead them.

Marje