I describe myself as a unilingual anglophone.
Apparently when I was young I could speak some Polish. Now I can tell when people are speaking Polish but there's no comprehension. As a Canadian I took French in school. However conjugating verbs and learning vocabulary never translated (no pun intended) into the ability to be conversant in French. I may be more aware of the French language than Polish but I never became bilingual. C'est la vie.
In high school, back in the 80s, I remember thinking that if I was to learn a foreign language (which ruled out French because in Canada it's not a foreign language - no matter what anyone West of Ontario says) I'd probably learn Russian or Japanese. Not that I did of course. Other priorities distracted.
I'm not sure what the 'best' languages are to learn to broaden one's horizons now. Politically and economically a case can be made for learning Chinese. Mandarin in particular. A case can be made for learning written Chinese or Pinyin. But now that I know how hard Chinese is to learn...
I'll think I'll just be happy being a unilinguist.