Language detect issue

This site is in English and French.

I had the auto-detect option on so it would show the language of the user on load.

But a user in English only got the home page in French on mobile – couldn’t tell why.

Then deactivated the auto-detect language feature and it’s back to English on load.

But we want to have auto-detect.

Is that feature not always working? What are the limitations? Any way to make it most of the time?

Is it based on location too? If in a region that uses both languages could confuse it?

How doe sit work?

Thanks

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