Or for another example, Serbo-Croatian used to be considered a language with Serbian and Croatian being just dialects. Then after Yugoslavia split apart into separate nation states, the Serbian and Croatian governments started insisting that they actually had separate languages. They even went to the extent of changing some words to increase the differences.
And yet, not only all speakers of one can understand perfectly the other, everyone can speak it effortlessly, and even understand localized accents and subtleties.