Don't check Plural-Froms if language is unknown
Don't check Plural-Froms if language is unknown

If the language header is not sent, don't even check the plural forms
expression and instead complain about missing language setting - that's
the more highlevel problem that the user can fix more easily.

Previously, Poedit would show a less understandable message about
invalid (e.g. default) plural forms value and wouldn't even alert the
user to the missing language setting.

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