In a way that has to do with language. If two countries are linguistically close, their languages are alike. Linguistically speaking is a slightly formal way to start a comment about language. The word does the same job as linguistic, only it tells you how or in what respect: not a linguistic fact (a fact about language) but linguistically interesting (interesting when you look at the language). It is a university-and-essay adverb; in ordinary talk people say in terms of language or when it comes to the language. It does not mean “in many languages” and it does not mean “literally.”