Langcode case mismatch causing exception

Problem/Motivation

Seboettg\CiteProc\Exception\CiteProcException: No Locale file found for pt-pt em Seboettg\CiteProc\StyleSheet::loadLocales() (linha 63 de /var/www/html/vendor/seboettg/citeproc-php/src/StyleSheet.php).

For European Portuguese, this:
$this->languageManager->getCurrentLanguage()->getId();
returns 'pt-pt'
and Citeproc expects the ISO code 'pt-PT', causing and exception.

Proposed resolution

Use a regular expression to make sure the format is correct.
Patch attached