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?
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.
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.