Thunderbird keyboard shortcut to switch dictionary language

Question

As a non-American I write emails in multiple languages. In the newest releases of Thunderbird the add-on Dictionary Switcher is no longer supported, so I would like to just add a shortcut for cycling through installed dictionary languages.

I am aware of the add-on Automatic Dictionary, but that only works on remembered contacts.

I have the KeyConfig add-on, but I can't find a working formula for changing dictionary language in the compose window. I have tried these three:

var SD='spellchecker.dictionary'; 
try{var sd=navigator.preference(SD); 
  sd=prompt('Spelling dictionary',sd); 
  if(sd!=null && sd!='') {
    navigator.preference(SD,sd);
  }
} catch (e) {alert('Permission to change the dictionary was denied');}

And

changeLanguage({ target: "en-US", __noSuchMethod__: function() { } }); 

And

Services.prefs.setCharPref("spellchecker.dictionary", "en-US");

Does anybody know code that works or potentially a different solution? I am on Ubuntu.