Question

What is the leaf for in the mouth of Pancham and Pangoro?

Answer 1

Pangoro's twig is stated to be bamboo in its Moon Pokedex entry:

From the slight twitches of its bamboo leaf, it deduces its opponent's movements. It's eager to tussle but kindhearted toward its companions.

From this, we can deduce that Pancham's leaf is almost certainly bamboo, as well.

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Answer 2

It's just a design choice. Moon's Pokedex entry sums it up pretty well:

There’s no point to the leaf in its mouth, aside from an effort to look cool.

The X and Omega Ruby entries also say it's Pancham's trademark. The leaf likely just carries over upon evolution. Pangoro's X and Omega Ruby entry claims:

It uses the leaf in its mouth to sense the movements of its enemies.

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Hope I helped!