This is like speciation but for languages: there's no "ah-ha!" moment, but we know a lemur can't produce viable offsprings with a zebra. Likewise we know Italian isn't French even though some words are kinda similar. If you want to be technical about it, it's a spectrum: I understand British people and people from the American deep South, but it's far from certain they will understand each other. Hard to be precise with social sciences.

That said, two people who understand each other are, by any reasonable definition, speaking dialects of the same tongue (if not, obviously, the very same dialect).