I don't know... Is it? Dutch I mean?
Nah languages branch off and diverge into different forms over time when separated from each other. Then, with all those forked off local varieties, modern states and institutions established standard forms from among the many that exist locally (typically the version spoken by the rich people in the capital).
So German, Dutch, English and Swedish and many others in the area are Germanic languages in a single family tree. Those forked off at different points in the last couple thousand years, local people generally communicated easily enough with their near neighbours.
Dutch and English are closer to each other than modern German. However, all three are standardised/prestigious ideals of a variety of dialect forms. There was/is a continuum of dialects and languages between standard Dutch and standard German. If you were to go from village to village between Amsterdam and Berlin or Munich, you wouldn't find two adjacent local varieties that were fully unintelligible from each other, but gradually Dutch would be turning into German as you travelled. If you went north from Amsterdam you'd instead move into areas where the local dialect is closer to English (called Frisian)
Slavic languages have similar forked tree relationships, Ukrainian and Russian and Polish are all different points in the branches of Slavic languages, and all three are standardisation established from different locations there (you'll be shocked to learn that the Kyiv and Moscow varieties form a lot of the modern standard versions there, though I think Polish is a bit harder to pin down on that front due no doubt to its very complex and fragmented process of state formation). Belarusian is another that's close to Russian and Ukrainian while Polish is closer to Czech and the languages in former Yugoslavia are another group. They're all versions of Slavic languages that developed over time from earlier ancestors, though.
Going back far enough the Slavic and the Germanic languages and others (like Romance) themselves are forked off from Indo-European, so there's ultimately a distant commonality between most (but not all) languages stretching from England to Persia and Northern India. The relationship is all modern languages developing simultaneously, though, rather than some modern languages being descended from other modern languages.
Languages rarely combine with each other, except in cases of forced prolonged contact in an isolated situation between groups with nothing in common language-wise. When this does occur, it happens through stages of pidgins and then creoles - basically people make do and construct a way to communicate simply with regularised and reduced versions of their languages. Then later on their kids inherit the result as a mother tongue, and turn it into a full new language. Most examples of contemporary languages that originally developed that way are the result of colonialism and slavery. Think of Haitian Kreyol or Jamaican Patois which are the majority languages in those countries and have official status alongside French and English, respectively. There's dozens of such examples around Africa, the Caribbean and the Pacific.