What mono is having or knowing only one language?

You're close! While "mono" does relate to "one," it's not quite "having or knowing only one language."

"Monolingual" is the word that describes someone who speaks only one language.

Here's how it breaks down:

* Mono- means "one"

* -lingual relates to "language"

So, a monolingual person is someone who is "one-language" or speaks only one language.