Learning another nordic language after Norwegian
Let's say, that somebody will learn fluently Norwegian.
How difficult it will be to learn Danish or Swedish or Faroese or Icelandic after that?
Maybe difficult question, sorry for that.
Icelandic and Faroese both have a noun case system - I know Icelandic has four cases, I think Faroese does too. So the grammar is about as complicated as High German.
It was my experience that Danish was pretty quick to pick up after Norwegian. Swedish wrecked my limited ability to spell in Norwegian and Danish. Swedish is more accessible to the English speaker, particularly in the areas of spelling and pronunciation. Danish pronunciation is challenging, to say the least.
Well, I'm nowhere near speaking it fluently, but even I can sometimes pick up a Danish or Swedish text and make sense of it. There are enough words that look/sound the same. I've also spoken to Danish people who have learned conversational Norwegian by just chatting to Norwegians for a year, without any study.
So, how hard is it? I don't know. Hard to quantify. But it's definitely easier than learning Swedish or Danish from scratch, apart from the "accidentally mixed up the two languages" factor...
I'm not fluent, but I think you can learn Danish and Swedish very easily after Norwegian, they are really close. Written Danish is almost like Bokmål but the pronunciation is quite different. For Swedish it's the contrary, the pronunciation is close but the spelling is a bit different (with the letters ö and ä for exemple).
Icelandic and Faroese are less related to Norwegian than the other ones. It's more like French/Spanish/Italian/Portuguese. From what I see about Icelandic, speaking Norwegian really help. But it's a harder language because of the grammatical cases like German (but certainly doable).