gh-511 Support Accept-Language header values containing country subtag

[kerkhofsd]

Issue #, if available: #511

Description of changes:

Support Accept-Language header values which also contain a country part.

Unlike mentioned in the GH issue, I did not use Locale#forLanguageTag to parse the language tag because it looks like invalid / unknown language tags result in an "empty" Locale object which I think might cause undesired side effects.

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• I confirm that I've made a best effort attempt to update all relevant documentation.

[deki]

Thank you very much for your contribution @kerkhofsd.