For example, how long have slavs known that all slavic languages are slavic languages?
In De vulgari eloquentia (1305), Dante Alighieri (author of the Divine Comedy) describes three classes of vernacular Latin: lingua di sì, lingua d'oïl, and lingua d'oc, corresponding to Italian, Old French, and Provencal, respectively. He recognized that there were dozens of dialects spoken in these three regions, but he sorted them according to their words for "yes."