localization bug fix

There is a bug (since 3.5??), localization strings are not found and then the javascript abords :

ReferenceError: setPostThumbnailL10n is not defined
$link.text( setPostThumbnailL10n.saving );

Here’s the fix :

https://pastie.org/5580662

https://www.ads-software.com/extend/plugins/multiple-featured-images/