WHY DOES IT DO THAT!!!?

It doesn't? I'm multilingual and always had 3 keyboard languages in parallel and never had any of those issues happen in 15+ years and 6 versions of Windows.

Most likely he was hitting alt+space by accident which cycles through them without him realizing.

Hot take: I feel like a lot of Windows issues people raise are actually user errors/accidents, then blaming the OS for it.

Systemic user error is a software defect.

How the user mashing random keys a SW defect? The SW does what you tell it to do.

Random mysterious keybinds that nobody asked for

Every OS has it's own keybinds. It's not their fault you don't know them.

I know the keybind to switch languages. I'm ok with it. I'm genuinely asking why Windows is adamant that I must have more keyboard language options than the two very specific ones I want.

If enough people are unaware that they're telling the software to do something and are confused by its behavior, then the software needs to better indicate what it's doing and why.

Could be ctrl+shift. It's easy to mistype, and there's no UI feedback. Also used to only change a single window's language (not sure if that's the case anymore, as I use win+space).