I've been looking for it for maybe thirty years. Read it in the eighties in the US in English. Probably published in the eighties. Standard novel length, call it 120,000 words? It was a space opera-type novel that involved, in part, the concept of a world of humans that had learned to assign children to the specialty/career they were suited for and would regard as a form of play even as adults. It was not a YA novel, though. Mercenaries from this planet were the best because they "played" at war with that loose ease we all feel when we're playing a game we love. This world also produced the best diplomats, the best engineers, the best specialists of all varieties. The specialist from this world were in high demand all across the galaxy. This novel really struck a chord with me, but I can't remember the title or the author and haven't had any luck searching for it.
It is the Regiment series by John Dalmas, and the "Play" is a part of the Matrix of T'Sel. The book is available on Baen, I'll look up a blurb.
This phrase summarizes the Regiment philosophy:
For T'swa mercenaries, war was an activity as pure as healthy sex, and apparently as satisfying. Eventually, through death or wounds, they lost the ability to play at war any longer. That would happen to him with sex someday, through death or age or whatever.
This is the part in which the importance of Play is stated:
"Each T'swa warrior," Usu continued, "becomes a master at the art of war before his regiment is ever commissioned, and performs his art as perfectly as he is able, without distracting himself with concerns of victory or defeat, survival or death. The warrior at Fight will usually fall to him. And for just those reasons, our regiments enjoy great success."
There are some similarities between the T'swa philosophy/tech and some doctrines in which John Dalmas was said to dabble.