Language preference auto-detection

[aapeliv]

Once the language preference cookie (#5469) and UI/toggle (#5470) are done, we should implement an auto-detection feature that suggests switching languages if the browser's language isn't the same one as the preference; or if the preference is not set.

[nabramow]

@aapeliv I wonder if this one is a bit over-engineered. There will be automatic language detection in next. If they don't want that language, they can select a new one with the dropdown or whatever we have.

Otherwise if I'm an American in Germany, for example, it's going to ask me this every time and that seems annoying.

I think we could just put a flag in the header of the current language really visible. The flag opens a dropdown, then it's easy to change.

But yeah, this ticket could be instead (detect when cookie changes on frontend and update i18n).