For an example, here, zh is not supported, but zh_Hans and zh_Hant
are. If the user asks for zh, we should choose a variant for them
instead of fallbacking to default.
Some browsers (e.g. Firefox) does not allow users to customize
their language codes. For example, there is no zh-Hans, but only
zh, zh-CN, zh-TW, zh-HK, etc. This provides a workaround for
those users suffering from bad design decisions.
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