wp-includes/class-wp-editor.php:1035Returns the TinyMCE locale.
string public static function get_mce_locale() { if ( empty( self::$mce_locale ) ) { $mce_locale = get_user_locale(); self::$mce_locale = empty( $mce_locale ) ? 'en' : strtolower( substr( $mce_locale, 0, 2 ) ); // ISO 639-1. } return self::$mce_locale; }Introduced in 4.8.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-editor.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.