My language is set to (US) English. I click a few words on a French page.
'propres' brings up a French definition, not an English translation 'propre: Qui appartient d'une manière exclusive ou particulière à une personne, une chose.'
Double-clicking 'permet' displays 'permit: Give authorization or consent to (someone) to do something.'
'invitent' gives 'No definition found'
'nombre' results in a Spanish definition, 'nombre:
Palabra o conjunto de palabras con las que se designan y se distinguen los seres vivos y los objetos físicos o abstractos.'

It hasn't been working for a long time now. It's been showing "Sorry No definition found." tooltip for ages.

Update: the extension stopped working lately. Sucks.
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Simple to use and it works. Double click on a word and you get a definition.