And even greater opportunity cost is the lack of in-depth knowledge of your own language.

Language isn't just grammar rules. It also has cultural/regional/historical/etc.

You can learn a few languages and be somewhat proficient at all of them or you can have expert level grasp of the language - where you can discuss philosophical, historical, economic, cultural, etc topics.

Not to mention, jokes, puns along with culture-specific references.

Personally, I wish the education system ( in the US ) focused on english, latin and ancient greek for K-12. I don't think you can fully understand english without understanding a bit of latin and even ancient greek.