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”God jul” is either/both Norwegian and Swedish. The two languages are mostly mutually intelligible; written Danish is as well but Swedes and Norwegians usually find Danish pronunciation… not so easy to comprehend.

With Icelandic there’s already much more guesswork and expertise required, mostly because the language has changed little since the days of Old Norse, and has not undergone grammatical simplification or adoption of (mostly Germanic, Romance) loanwords nearly to the same degree as the others.