Use index of the supported tags to choose user lang (#15452) #15488
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Backport #15452
Fix #14793.
The previous implementation used the first return value of matcher.Match, which is the chosen language tag but may contain extensions such as de-DE-u-rg-chzzzz.
As mentioned in the documentation of language package, matcher.Match also returns the index of the supported tags, so I think it is better to use it rather than manipulate the returned language tag.
Co-authored-by: Naohisa Murakami [email protected]