It seems a stretch to call having a + character at the start of an added line “its own language”. Staging one line of the two in your example probably takes less than five seconds using this method; in the editor, you just delete the line you don’t want.

If you personally prefer a GUI then that’s obviously your choice. Some people do and I doubt you’ll find many people claiming Git has the world’s best UI. It does work everywhere, though, and I doubt it’s any slower than using a GUI for a task like your example. Either way, you can have lines staged or not pretty much as fast as you can read them and decide whether they should be.