This is an excellent complaint: most software products don’t work well for people that switch languages frequently (or mix languages!). I’m effectively monolingual so I’m part of the problem, but I have experienced this pain over the years when developing and testing localized software — it’s a pretty big hassle to switch. MacOS is the easiest — just drag your preferred language to the top of the list — but I agree even that’s too much of a hassle for things like dictation, autocomplete and autocorrect.
http://www.osnews.com/story/29950/The_lack_of_multilingual_affordances_in_modern_software