about toddlers – in my experience they don’t understand what you are saying when you speak their ‘language’. For example my younger sister called my sister a nickname instead of her real name cuz she couldn’t say it. But when we would ask my baby sister if she wanted the other sister using the nickname she would have no idea what we were talking about.
about other people who don’t speak English properly – I don’t think it’s a conscious thing. But it could be you just adapt how your talking to theirs without even realizing.