Unfortunately, I find the message not so helpful. First of all, there are 2 ways to solve this: Either reconnect (to the element in other language) the unconnected content elements or unconnect them all.
What I found, the best bet is to switch to “List mode” where you can see better which elements are connected and which are not and which course of action should be taken.
If you do want to reconnect them you can do so in the “Language” tab.
Are there better ways to handle this?