The best rule is that location != language. Just like in UX a flag != language. Sites should honor the 'accept-language' header with the one exemption below.

The best rule is that location != language. Just like in UX a flag != language. Sites should honor the 'accept-language' header with the one exemption below. In various regions users do have their devices set to some version of English (localization not be available in their language or of poor quality etc.) so 'accept-language' will likely be set to English. In those cases (likely language for country != English), I think it's fine to _ask_ if they want to change to the expected local language. But in the end, if a users selects a language preference at any time, it should be saved to localStorage or in a cookie and respected even if other conditions change.