If you don’t want your spellchecker dropdown to be too big, you can limit the available languages. Cut it down to only a few, or even just one.
add_filter('mce_spellchecker_languages', 'limit_spellchecker_languages');
function limit_spellchecker_languages() {
//original: +English=en,Danish=da,Dutch=nl,Finnish=fi,French=fr,German=de,Italian=it,Polish=pl,Portuguese=pt,Spanish=es,Swedish=sv
return '+English=en';
}