Use index of the supported tags to choose user lang (#15452)

Use index of the supported tags to choose user lang (#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.