German umlauts and es-zett(sharfes s, sharp s) character on linux mint with german language keyboard layout.

german_keyboard_layout.txt

System Settings -> Keyboard -> Layouts

Add a layout with the plus sign, and select the German language from the list. 

Then you have a little flag icon in the system tray. (if in the options there are enabled using country flaf)

Now with a simple click You can toggle between your favourite lanuages.

You can check the Character Table, if you click on the language flag on the tray, 
and click 'Show Character Table'. 

You can check the Keyboard Layout, if you click on the language flag on the tray, 
and click 'Show Keyboard Layout'.