the language tool in Ubuntu 16.04 is defective at the moment so please wait a short while until it is updated and fixed!.
I use German and English on a daily basis myself and I have problems with the tool myself, in the meantime; you can take a look at the language guide but as far as I am aware, there is no flag setting for Ubuntu or Ubuntu Mate!, take a look in Dconf Editor for a setting that states “Show flags” or something like that but I cannot find it !. 
How to change the language settings in Ubuntu
Zur Zeit ist die Sprach Tool defekt, Bitte Geduld haben bis einen Fix Vorhanden ist!. 