Media: Further improvements to the handling of language codes that get passed to MediaElement.

This change means that only the leading portion of a locale code gets passed to MediaElement, removing problems that arise from locales such as `de_DE_formal` and `pt_PT_ao90`.

Props erich_k4wp, blobfolio, flixos90, ocean90, joemcgill, SergeyBiryukov.

Fixes #42574

Built from https://develop.svn.wordpress.org/trunk@42251


git-svn-id: https://core.svn.wordpress.org/trunk@42080 1a063a9b-81f0-0310-95a4-ce76da25c4cd