The ' Save, Undo, buttons, search box and Language toggle ' will return in an upcoming Mendix version. See this idea thread.
Which toolbar do you mean?
If the toolbox isn't available anymore in the editor, you can reopen it via the view menu item
I don't mean the Toolbox with the tools; I mean the Toolbar below the menu bar. I didn't find a menu item to open it.
It's gone. Use shortcuts for example Save All Ctrl+Shift+S. See all here https://docs.mendix.com/refguide/desktop-modeler-overview