I finally found the solution: (close Mozilla Thunderbird then)

1. for the English and Spanish dictionaries: go to /usr/share/hunspell/ and remove everything but
   
   
   
   
   • en_US.aff
   
   
   • en_US.dic
   
   
   • es.dic
   
   
   • es_ES.aff
   
   
   • es_ES.dic
   
   


2. for the French dictionary: go to ~/.thunderbird/***.default/extensions/[email protected]/dictionaries/ and remove everything but
   
   
   
   
   • fr-modern.aff
   
   
   • fr-modern.dic
   
   

Obviously, instead of actually removing those entries, you can create a folder named "hide" and put the entries you don't want in it.

Now when you write a email and right-click, it only shows up English (US), Spanish (Spain), French (modern)