IETF / ISO-639 mixed case language code support

[sysoce]

Problem:
Uppercase language codes such as Traditional Chinese "zh-Hant" is converted to lowercase "zh-hant".

Suggested solution:
Add mixed case support for language codes and URLs.

Possible related to #1407

[bep]

Problem:
Uppercase language codes such as Traditional Chinese "zh-Hant" is converted to lowercase "zh-hant".

Why is this a problem?

[stale]

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[github-actions]

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