locale_get_plural() only works for a single language within a request

Problem

Calling the function locale_get_plural() with different language codes, will give you a notice and incorrect return values:

Reproduce:

locale_get_plural(3, 'nl');
locale_get_plural(3, 'fr');

Expected:
1
1

Actual:
1
Notice: Undefined index: nl in locale_get_plural() (regel 743 van modules\locale\locale.module).
-1

This because the check is on the static array, not its entries and isset should be used instead of (implicitly) converting to bool.

Proposed resolution

I will attach a patch in a comment.