I didn’t understand most of the conversations around me when I was in France for my Christmas break. If I had learned how to read and write and pronounce words in French, I know I would’ve understood more because many French and Spanish words are lexically similar (as opposed to phonetically).
Anyway, to answer the question posed in the title, one of the most rewarding ways to increase your English vocabulary is to learn a Romance language. For example, I heard some SAT vocabulary words flying around in normal conversations in France. Laissez moi, I heard Elodie say. I understood her because of Laissez Faire. I also heard Elodie’s mom say ennui at breakfast, which I learned from Mr. Mell’s English class.
Other than the Latin languages, German and Greek have also greatly influenced English; its dynamism and flexibility are probably the reasons for irregular verbs, different pronunciations and spellings, and exceptions to grammatical rules. However, this diversity of the English language is also what make it so rich and interesting.
Having said all of this, one must be careful when translating similar-sounding words.
The French sell products of terror and pain and rape.