This is the 3726th most frequent Spanish word.
portugués
"Portuguese" (language or nationality).
Here, 'portugués' is used as a noun to refer to the Portuguese language.
Él está aprendiendo portugués.
He is learning Portuguese.
In this sentence, 'portugués' functions as an adjective describing the nationality of the subject.
Mi mejor amigo es portugués.
My best friend is Portuguese.
'Portugués' is again a noun here denoting the name of the language being taught in school.