Locale settings expressed by the browser are the user's explicit preference,
From experience watching other people use computers, most people don't even know WTF "locale" is or anything even tangentially related. More than likely they just use whatever was configured by default, which most of the time makes sense for them but it's not a given.
Hence, it's unreliable.
IP geolocation on the other hand, you can at least assume the broad region where they are connecting from (VPNs and proxies aside).
That "default" is typically set by the language choice in the OS, which is exactly why it is reliable, because people understand language selectors just fine.
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From experience watching other people use computers, most people don't even know WTF "locale" is or anything even tangentially related. More than likely they just use whatever was configured by default, which most of the time makes sense for them but it's not a given.
Hence, it's unreliable.
IP geolocation on the other hand, you can at least assume the broad region where they are connecting from (VPNs and proxies aside).
That "default" is typically set by the language choice in the OS, which is exactly why it is reliable, because people understand language selectors just fine.
I know precisely zero people who know how to change the OS's language settings, assuming they weren't already knowledgable with computers.