If `$locale` is invalid, the return value is actually the value of `$locale`, not `NULL` or `FALSE` as you might expect.
(If `$in_locale` is invalid, but `$locale` is valid, the return value is the language name in the default locale.)
If `$locale` is invalid, the return value is actually the value of `$locale`, not `NULL` or `FALSE` as you might expect.
(If `$in_locale` is invalid, but `$locale` is valid, the return value is the language name in the default locale.)