Well, I'd say the song list should be geared towards the audience/participants, not only in terms of their geograpy/nationality but also their ages. I do sing-alongs often for senior citizens - mostly over 80 - in retirement communities, but there are often younger staff members and/or family participating, and sometimes even some of the residents are boomers or a bit younger.   Here are some of the songs that seem to go over well in such settings, as well as in many folk music community sing-alongs. They may not all be everyone's favorites, but usually most everyone can sing along and they are often frequently requested.

You Are My Sunshine
Take Me Home, Country Roads
Loch Lomond
Ain't She Sweet / Five-Foot-Two
Help Me Make It Through The Night
Hey, Good Lookin!
This Land Is Your Land
If I Had A Hammer
Red River Valley
Sloop John B
Jamaica Farewell
Ring Of Fire
I Walk The Line
Your Cheating Heart
San Antonio Rose
This Little Light Of Mine
Shine On, Harvest Moon
Puff, the Magic Dragon
The Marvelous Toy
Rambling Boy
Beer Barrel Polka
Shenandoah
Guantanamera
Mama Don't Allow
La Bamba
Over The Rainbow
Scarborough Fair
Simple Gifts
Garden Song ("Inch by Inch")
Home on the Range
Oh Mary Don't You Weep
Down By The Riverside
Go Down, Moses
Moon River
Yesterday (Beatles)
Let It Be (Beatles)
All I Have To Do Is Dream
Stardust
Paradise (John Prine)
Annie's Song (John Denver)
Me And Bobby McGee
Ripple (Grateful Dead)
City Of New Orleans
House Of The Rising Sun
Java Jive
O What A Beautiful Morning!
Que Sera Sera
Kisses Sweeter Than Wine
Leaving On A Jet Plane
The Gambler (Kenny Rogers)
Could I Have This Dance (For The Rest Of My Life)?
Little Boxes
Morning Has Broken
Proud Mary (CCR)
Under The Boardwalk
Amazing Grace

As I said, it varies with the age of the group. When there's a wide age range in the group and/or a wide range of musical tastes, I'll include some songs which mainly the elders will know (e.g., Down By The Old Mill Stream or Sentimental Journey) and some which mostly the relatively younger ones will know (e.g., Rubber Ducky, from Sesame Street). ;D