Hi Kaio,
It does look like Somali is on the supported list but not for Custom Translator.
https://video2.skills-academy.com/en-us/azure/ai-services/translator/language-support
However, I do not see release notes for it ever being a part of Custom Translator. All I see is:
I'd recommend following the Release Notes (link above) for when Microsoft releases this feature.
If this is helpful please accept answer.