Source

File: wp-includes/class-wp-editor.php

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;
	}