Use conformant language tags

[ghost]

https://github.com/spf13/hugo/search?utf8=✓&q=en-us

This seems like such a low priority, persnickety thing I've never opened an issue. But I am now. Perhaps something good for a first time contributor who wants to learn about language and how it's used on the internet.

Quoting myself from the following:
jamstack/jamstack.org#56 (comment)

To nail spec for language tag identifiers from RFC3066 simply change this to en-US, and the UK config to en-GB or en-GB-oed if language variant used. The example locales in the Hugo docs are not currently compliant with spec. Not confident this change will help with achieve what is sought here, but could be helpful in eliminating variables nonetheless. Cheers.

http://www.iana.org/assignments/language-subtag-registry

[stale]

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