For the Dragon of eternity

Garen flapped his dragon wings.

Along with his movements, the surging Firebird was like a storm, pressing the flames forward. It was as if the flames had formed layers of waves, and it was like a powerful fire-type spell that swept through flame Steel City.

At the same time.

The flames didn't affect the fire elementals in the city much, but when the Firebird swept past, the fire elementals in the city trembled, like sparks swaying in the wind.

"Great Eternal Dragon, flame Steel City welcomes your arrival."

the fire spirits and fire elementals pay their respects to you.

The fire giant spirits in the city knelt on the ground and lowered their heads to show their respect to garen.

As for the language they used ...

It was dragon language.