Fix lang() signature

[jim-parry]

lang() indeed returns a string or array of them.
Fixes #2075

[klikklak]

Thank you for fixing the signature. There is still a little «bug» in the the comment:
* @param string|[] $line
* @param array $args
* @param string $locale
*
* @return string

=> this should be like that:
* @param string $line
* @param array $args
* @param string $locale
*
* @return string|[]