Language usage is dependent on Itemid (Joomlas language filter then adds the appropriate language). We do not construct the URL using CBs API, but using UddeIM API. We directly pull the Itemid from uddeIMgetItemid API call. Reviewing the latest UddeIM release our URL construction is 1:1 exactly the same. Issue would be in UddeIMs API. Specifically uddeIMgetItemid or misconfigured menu items. Ensure you've a Joomla menu item to UddeIMs inbox that's specific to each language.