How to switch input language with good old Cmd + Space in Sierra?
⌘+␣ and ⌘+⌥+␣ are now used to "Show Spotlight Search" and "Show Finder search window" respectively.  You will have to change those shortcuts before you can change these.  Click on spotlight in the left pane, double click on the shortcut and key in your new combination to change it.

Here's proof that it can be done.


For the basic Apple instructions, see this help article.