All foreign names and words with (S) letter which are entered into Persian are written in (س) . If we borrow a word from Arabic, we will write it exactly same as Arabic letters like( ص or ث ) otherwise we just use (س ) for all Persian and Latin words etc.
We Iranians prefer to derive names from Indo-European languages rather than Hamitic and Semitic ones therefore because of this attitude our letters are different from them in the most names.
The Persian name of Somalia is derived from the French Somalie, from the name of the native people Somalis, perhaps from the Cushitic language "dark", referring to their skin color, and perhaps from the phrase soo maal "to go and drink (drinking) milk".It refers to their hospitality with milk; And maybe it is from the name of their tribe's chief that is Samaale .
By the way somal ( صومال ) has different meanings in Arabic which comes from Afro-Asiatic root .