As wise men say 'there are two schools of thought' about it. Some time ago there was ONE language. Then, because communications between men in different regions was not as easy as it is now, they started to separate. New words were borrowed from neighboring nations. Technically, even Polish is somewhat understandable to a Russian speaking person, same with the whole Slavic family of languages. The stems of the most ancient (core) words are the same. There are rural people in Ukraine who speak neither Ukrainian nor Russian but their mix.
Similarly, my opinion is that Swedish and Norwegian are technically two dialects of the same language as well.
In Europe you can find 'mixtures' of languages like in Netherlands where they speak a cocktail of English and German. Russian immigrants in USA speak a mind-blowing mixture of English and Russian.
And finally, that's no big secret that nearly all European languages derived from the same proto-language.
The question is - where to draw the line between one language and another?
