This is the 10142nd most frequent Catalan word.
parleu
"Parleu" means "you speak" or "speak" (second person plural, imperative or present tense) in Catalan.
Here, "parleu" is used as the present tense in the second-person plural (vosaltres) to indicate that the subject 'you all' are speaking Catalan.
Parleu català a classe.
You (plural) speak Catalan in class.
Here, "parleu" is directed as a negative command in the second-person plural (vosaltres), instructing more than one person not to speak.
No parleu mentre explico.
Do not speak (plural) while I explain.
In this question, "parleu" is used to inquire about the language habitually spoken by a plural subject, formulating the second-person plural as the subject of the inquiry.