(due to a comment by @Kout)
This is likely due to a bug with Cinnamon - not a setting which hides the indicator... which is why you can't just tell Cinnamon to make the keyboard layout indicator appear again, directly.
But - the bug has a workaround:
- Run Cinnamon's launcher search box, type "Keyboard"
- Choose the single app named "Keyboard" and nothing elsestart-menu).
- At the top of the window, press "Layouts"
- You'll see a list of layouts; press the '+' button at the bottom to add a new layout.
- Select a layout you don't already have on the list, and confirm to exit the sub-dialog.
- At this point, the layout indicator should re-appear on the task bar!
- Select the layout you just added, in the list
- Press the '-' button to remove it
Here's a screenshot of the Keyboard settings window with "Layouts" selected:
