Personally, for a launcher application, I would use Qt - or another cross-platform application framework if I didn't know c++. There's PyQT for Python, for example.
Launcher applications typically require a whole bunch of functionality (download management, file reading/writing, patch notes, maybe mod management, etc.) that is best served with a UI framework like that and not a game engine IMO.
They have far more in common with "normal" user applications than with games.
Of course, if you want a highly stylized launcher (displaying actual 3D graphics), you'd be better off using the game engine.