From Joomla! Documentation
Managing multilingual sites is since Joomla! 3.7 far easier thanks to the Multilingual Associations feature. The Multilingual Associations Component allows you to translate your content from one interface.
The Multilingual Associations Component is a tool that enables editing associated items side-by-side without the need to go back and forth, for example create the item in several languages and then associate them.
The side-by-side view is the backend screen where you can edit items after selecting a reference item. You can switch the target and apply the changes you desire without going back and forth. This will also preserve existing associations and will create new ones on the fly.
This feature was done during the Google Summer of Code Project Improved Multilingual.