[norman784]

Well it's a game engine and the language that is based on is important, you would for sure pass on a game engine written in Javascript and instead you will choose one that uses technologies you know.

So I think is important to explicitly tell which technologies are you using in a game engine.

[pjmlp]

Depends, PlayCanvas, BabylonJS and ThreeJS are written in JavaScript.

Although you might say that there is still the C++/Rust code of the browser rendering engine to take into account.