Knowing multiple languages, in particular less related or unrelated ones, is mentally enriching. If you only know a single language, you don’t know what you are missing in terms of diversity of thought and expression. Languages aren’t fungible. Translation is lossy, often considerably so.

>Knowing multiple languages, in particular less related or unrelated ones, is mentally enriching

Just as being able to play an instrument or dance or actually reading literature from other cultures, even in English.