Update: After much head scratching, I think “calm” is a mistake on the marketing team’s part. Listening to the CV, the name sounds like “Karum” and not like how a Japanese speaker would say “calm”. A quick google search revealed that “Karam” is an actual Arabic name meaning “generous”. This would be more inline with the naming convention in the series. So… I’ll go with “Karam” since this is most likely the intent.
Katakana is always a struggle. In fiction it’s always difficult to tell the mangaka’s intent because there are so many homophones and often times the mangaka just makes up a name for a person, place, or thing based on how it sounds. With regards to “Sabaku no Harem”, it appears that the mangaka’s intent is Prince “Calm” instead of Prince Kallum. When I started the series I was wavering between Kallum and “Karum”, but there’s now an official twitter account for the series called “Calm Harem”, which is a possible translation of “カルム”. So from now on, I will translate as “Prince Calm”. The name makes me wince because it’s too on-the-nose, but this is nothing new because most of the time character names are on-the-nose in manga and anime.