Selamat siang translation
There is no single equivalent in English as English greetings split at noon between morning and afternoon. Duolingo seems to refer to afternoon for siang. Selamat malam also returns 'evening'. English users would say good evening when arriving at night, but good night when leaving. Sometimes custom dictates which expression is used rather than direct translation.
The unspoken nuance of "Selamat siang" is roughly equivalent to what English speakers mean by "Good afternoon" even though culturally "Selamat siang" is used later into the the day than "Good afternoon."
One course I use breaks it down like this:
Selamat pagi - used in the morning until 10 AM
Selamat sore - used from 10 AM to 2 PM
Selamat siang - used from 2 PM to 6 PM
Selamat malam - used after 6 PM
That is a rough breakdown to help English speakers grasp the cultural understanding of how a typical day is seen in Indonesian culture But of course it's a bit more nuanced than a time schedule.
Selamat pagi refers to the beginning of a day, when people are just starting their daily routines.
Selamat sore refers to the early part of day, when people are typically well-engaged in their daily routines.
Selamat siang refers to a later part of the day when people are nearing the end of their daily routines and are transitioning to evening routines, like going home or having an evening meal
Selamat malam refers to the later evening/night time when people are done with their daily routines and would be relaxing, socializing or getting ready to retire to bed.
These greetings are more about the activities people are doing throughout the day than the clock on the wall. And no, English greetings do not correspond exactly with that because English greeting reflect the actual time of day more than people's routines throughout the day.
English greetings however do carry some nuance that respects people's daily routines--"Good evening," for example, carried the connotation that we might be imposing on someone's own personal time for resting and relaxing after a day's work. This is similar to "Selamat malam."
Meanwhile, "Good afternoon" carries a connotation that such an imposition isn't as bad because people are still engaged in their daily routines like work or school, and not on their own personal time. This is similar to "Selamat siang."
The big difference here is in English, greetings and when we are suppose to use them do conform more strictly to how we break down the day into morning, noon, afternoon and evening times according to the clock. This simply reflects a cultural difference in how we think about time between English speaking cultures and Indonesian cultures.
One way people often talk about this difference is using the terms "linear time" versus "cyclical time." Cultures that think in more linear time value things like strict time schedules. For example, when an English speaker says they will meet you at 9 AM, they mean literally 9 AM, on the dot. But for someone whose culture respects the concept of cyclical time more, when they say they will meet you at 9 AM, they often mean something like, "I'll meet you during that day of time"--meaning 9 AM is just a rough point in a general time they intend to meet you. They are thinking more of meeting you during a time that corresponds with their daily activities during that part of the day.
Here's an article that explain this theory of cultural difference between linear and cyclical time a bit: https://hbr.org/2016/05/different-cultures-see-deadlines-differently