Toggle language by pressing Shift + Command
With freshest version (11.4.0 at 13.12.2017) of Karabiner Elements and Cmd + e for previous language source:



You can put this json gist to ~/.config/karabiner/assets/complex_modifications/&lt;filename&gt;.json, add gist rule in Karabiner Elements Preferences -&gt; Complex Modifications -&gt; Rules -&gt; Add rule and Enable Languages -&gt; Command + Shift to Command + e.



This will remap Cmd + Shift to Cmd + e and you can change language source by Cmd + Shift.