Norwegian, Swedish and Danish are mutually intelligible languages. Your challenge is to make these languages one. Also, Iceland can't speak a different language. You can use any POD you want.
Also, Iceland can't speak a different language.
Well... it wouldn't require ethnically cleansing the inhabitants, though it would involve an infringement on their rights. But there are numerous cases throughout history were a language group has been forced to speak another language. It doesn't even need to take more than a handful of generations. You don't have to kill a single Icelander to kill Icelandic, you just have to set up an oppressive situation where they have to speak another language.Without ethnically cleansing the Icelanders or some other horrendous crime against humanity they're going to continue speaking Icelandic, as they have been for a millenia.