Where is edit summary?
There no way to add edit summary with this new interface. Edit summary is a very important feature in a wiki and it should be always availiable. The only workaround I can think of is (on an translated message only) to open the small dropdown list next to message, select history, and then from history select the message again, which takes you to the old interface. But this is too much to ask from fellow translators, especially when I already have strongly insisted to them to fill in some edit summary in the first place.
The relevant bug report is at bugzilla:46924, I hope someone will be able to give you updates,