те, кто счастливы/довольны

контекст давайте

This word doesn’t exist. If it did, it would be happies, according to her English is written.

Adjectives used as nouns to refer to groups of people are not pluralized: the poor, the rich, the sick, the happy…

If it doesn't exist one can invent it (like a name)
Cincinnati Reds
the Greens

The Happys and the Nesses
In a story about two kinds of children, there are the Happys and the Nesses. Happys are very happy, of course, but Nesses are, well, rather sad.

He tells them to look for Little Happys -- small things that bring them joy

That’s not English.

The names of sport teams are always pluralized.

A team isn’t «те, кто…»

Well, and that wasn't a translation. And that was why I asked the context.

That’s not English.

https://www.google.co.uk/books/edition/The_Happys_and_the_Nesses/-VekngEACAAJ?hl=en

Anyone can see that’s an isolated case of a children’s book using silly English.

Again, that’s not English!

А кто с этим спорит?
Уж точно не я.

one should consult some hippies on the subject

Well, since we’re talking about adjectives used as nouns to designate groups, and the adjective hippy means having big hips, I don’t think your logic applies here.