I mainly type in English but occasionally other languages - I use a combination of Mac & Linux - macOS has an (off-by-default but enable-able) lang-changer icon in the tray that is handy enough, but still annoying to have to toggle. Linux is much worse.
Mac also has quite a nice long-press-to-select-special character that at least makes for accessible (if not efficient) typing in multiple languages while using an English locale. Mobile keyboards pioneered this (& Android's current one even does simultanous multi-lang autocomplete, though it severely hurts accuracy).
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> I doubt many English speakers care to distinguish between English dialects.
I think you'll find the opposite to be true. US English spellings & conventions are quite a departure from other dialects, so typing fluidly & naturally in any non-US dialect is going to net you a world of autocorrect pain in en_US. To the extent it renders many potentially essential spelling & grammar checkers completely unusable.