Widget editing not possible if locale language code exceeds XXX_XX

[mkmyuu]

The new MediaElementPlayer causes disfunction to the Widget Customizer page (Appearance > Widgets) whenever the language does not match the XXX_XX pattern, for example de_DE_formal (German formal).

Errors:

TypeError: Language code must have format 2-3 letters and. optionally, hyphen, underscore followed by 2 more letters
mediaelement-and-player.min.js:320:10

ReferenceError: MediaElementPlayer is not defined
mediaelement-migrate.min.js:1:342

TypeError: b(...).not(...).filter(...).mediaelementplayer is not a function
wp-mediaelement.min.js:1:453

[mkmyuu]

Just noticed: Same goes for the Customizer (Appearance > Customizer).

[johnbillion]

Thanks for your bug report, @mkmyuu! Welcome to WordPress Trac.

We're already tracking this issue in #42574. It's scheduled to be fixed in WordPress 4.9.1.

You can follow that ticket by clicking the star next to its title.