[Tor3]

In addition to what vidarh said - it's not comparable to Mandarin vs Cantonese, as those are spoken languages. Nynorsk is one way of writing (and reading) Norwegian, it is not a spoken form (nor is Bokmål). Some dialects are kind of closer if you try to "speak" it (read out loud, from e.g. a book), and some are farther away, most dialects contain elements where some words will fit better in one form and other elements fit better in the other form. But even grammar is often different in spoken Norwegian (and spoken Norwegian is just a huge continuum of dialects, which, if you don't stop, stretch across the borders to Sweden and Denmark too).

[actionfromafar]

Sweden, Norway and Denmark is a perfect example of language continuum. At the fringes, the division between them is clearly for political and administrative purposes. It called all have been named "Danish".

I think what goes for "German" has even more internal differences.

[Tor3]

Yes, it's just a huge continuum. I call it "Scandinavian" for short.

[komadori]

You can write Cantonese, although most Cantonese speakers' "formal" written language is more closely related to spoken Mandarin (Modern Standard Chinese) than to the way they normally speak.

I recommend "Cantonese as Written Language: The Growth of a Written Chinese Vernacular" by Don Snow.

[IlliOnato]

How well people speaking different dialects understand one another? Is there some "common dialect" that everybody uses with speakers of dialect different from their own? What TV presenters use? Or nation-scale politicians?