I wouldn't say "Good evening" and "Good afternoon" are interchangeable in English, other than maybe a fuzzy area between 4pm and 5pm, but in Danish, eftermiddag is used between 1pm and 5pm and then aften is used between 5pm and middnight (or until one goes to bed)

For afternoon you could say: "god eftermiddag" in Danish.

Note that, unlike morning, evening and night, this phrase is not common enough to be written in one word, but it is still perfectly usable.