It has a dedicated key with a globe symbol in the lower left corner. I know what pop-up you are talking about, but that's non-input-blocking, i.e. the language

It has a dedicated key with a globe symbol in the lower left corner. I know what pop-up you are talking about, but that's non-input-blocking, i.e. the language is active when you press the button, and you can continue typing without any wait. It's possible that your iPad is not performant enough to both render that popup and and deal with keystrokes at the same time. I'd recommend the Pro iPads for any kind of serious work.