get_user_locale

function get_user_locale( $user_id = 0 ) {
	$user = false;
	if ( 0 === $user_id && function_exists( 'wp_get_current_user' ) ) {
		$user = wp_get_current_user();
	} elseif ( $user_id instanceof WP_User ) {
		$user = $user_id;
	} elseif ( $user_id && is_numeric( $user_id ) ) {
		$user = get_user_by( 'id', $user_id );
	}

	if ( ! $user ) {
		return get_locale();
	}

	$locale = $user->locale;
	return $locale ? $locale : get_locale();
}