If you have multiple keyboards (languages) enabled on iOS 4 under Settings > General > Keyboard, you can tap and hold on the world icon that appears to the left of the space bar to swap between keyboards on the fly.
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Little features that could make a big difference in iOS and iPadOS
I have a growing list of little features that I hope Apple will add to iOS someday. Since WWDC 2021 was not that day, I’ll share them here.