>differences of German [...] are much more significant than the differences between various forms of Serbo-Croatian.
Maybe, but it adds no argument to the issue of the same lang vs different langs. There's no clear purely linguistical distinction between dialect and language. As an old joke goes a language is a dialect with an army. In other words when it has a separate political status, and governed by a separate body - it's a language, no matter how similar it is to other languages. The reason is that a literary/official language is defined by rules, and as rules are defined by separate entities, existing differences will be preserved, and new will emerge, and multiply eventually.
It does only if you prefer to take the "army" part absolutely literally. Which you shouldn't because, as clearly stated, it's a joke. Literary German is governed by the one regulatory body - Rat für deutsche Rechtschreibung - recognized not by three, but seven European countries. And this what makes German one language. If tomorrow, say, Liechtenstein decides it's not going to follow the same rules, there will be two.
Maybe, but it adds no argument to the issue of the same lang vs different langs. There's no clear purely linguistical distinction between dialect and language. As an old joke goes a language is a dialect with an army. In other words when it has a separate political status, and governed by a separate body - it's a language, no matter how similar it is to other languages. The reason is that a literary/official language is defined by rules, and as rules are defined by separate entities, existing differences will be preserved, and new will emerge, and multiply eventually.