this is what i have done ...
joomla plugin "system-languagefilter" connecting is ON
i have a connection between the german and the english article
i have a connection between the german and the english menu-item
if you want to try out:
dodge-ram.fans
click on register
click on terms and conditions
you will find the german article in the popup
i have the normal site-template actualy in the popup. so you can change to english to see that they are connected