Short and dirty workaround for this Bug in TYPO3 Blog Extension TYPO3GmbH/blog#158
i'm using a Viewhelper, based on f:variable and call it CountPostsViewHelper and run through {categories}, count the attached posts from current Language and save the result in a fluidVariable In My Widget/Categories.html, add the namespace and use the viewhelper in the <f:for> loop
<html data-namespace-typo3-fluid="true"
xmlns:f="http://typo3.org/ns/TYPO3/CMS/Fluid/ViewHelpers"
xmlns:blogvh="http://typo3.org/ns/T3G/AgencyPack/Blog/ViewHelpers"
xmlns:abc="http://typo3.org/ns/MyNameSpace/MyProject/ViewHelpers"