> German-speaking Swiss share their written language with Germans ... but they are perfectly capable of talking in a way that is 100% indecipherable for Germans
You are talking there of a proper language on one side, and a bunch of non-codified dialects who never got any written form.
Yet those dialects tend to deviate from the standard language in very regular ways, even if they are perfectly incomprehensible to the uninitiated. When some new term appears in the standard language, speakers sharing the same dialect will independently shift it in the same way.
You are talking there of a proper language on one side, and a bunch of non-codified dialects who never got any written form.