Countable plural nouns & subject-verb agreement

A friend of mine claims that a sentence like:

Foreign languages is what interests me.

is the only correct option, whereas using the plural are would be wrong.

I'm not convinced about this and I rather feel that the opposite of his claim is true. Which formation is correct? And where could I find information that explains the rationale behind this?

UPDATE:

I tried to search similar examples online but I could only find ones which use a plural form there:

Spoken languages are what they are by virtue of their verbal, not their written, manifestations.

source: https://www.britannica.com/topic/language/Physiological-and-physical-basis-of-speech

However, proto-languages are what many linguists agree on.

source: https://www.thegreatcoursesdaily.com/proto-languages-and-their-evolution/ (John McWhorter, Ph.D., Columbia University)

Languages are what make us get up in the morning (and coffee, or a mug of Cadbury's hot chocolate!)!

source: https://www.superpolyglotbros.com/who-we-are/ (Two English guys from Manchester)

Languages are what makes us unique and different.

source: http://www.bbc.co.uk/voices/yourvoice/language_ecology.shtml

The real issue is whether foreign languages can be, or even should be, treated as singular. To better resolve the issue, I suggest using a simpler sentence: