Godot

Game Engine https://godotengine.org/ Godot is completely free and open-source under the very permissive MIT license. No strings attached, no royalties, nothing. https://en.wikipedia.org/wiki/Godot_(game_engine)

But Lorien (not a game) is also built in it.

Godot is built in C++.

can build mobile games, too

Godot has its own built-in scripting language, GDScript,[15] a high-level, dynamically typed programming language which is syntactically similar to Python. Unlike Python, GDScript is optimized for Godot's scene-based architecture and can specify strict typing of variables. Godot's developers have stated that many alternative third-party scripting languages such as Lua, Python, and Squirrel were tested before deciding that using a custom language allowed for superior optimization and editor integration.

good for other apps, too?