I used to pick up alot of Somali and spoke that. Lots of my relatives spoke basic English and we spoke normally.
But as I went out and played I picked up enough somali to help me survive at least but sadly I didn't stay long enough to refine it and become fully fluent.
I only have somali relatives. The Afars and Arabs I met spoke somali. People forget that year french is used in government and Arabic is used for religion but on the ground and everyday life. Despite it being diverse everyone in the city speaks Somali. You cannot survive in Djibouti city without Somali.
The cadaans I knew spoke somali as well. There's this french owned bakery next to my street and the cadaans spoke somali. As did all the Yemenis I met. I went to an American summer school and the students didn't know much, all the cadaan teachers spoke perfect somali and regularly interacted with locals. The headmaster was friends with my uncle who used to pick me up and the white American and my Adeer would speak for like an hour in af somali.
There's lots of people from all backgrounds you will see in certain parts of Djibouti city and they all speak Somali. However it is expected since its above 70% somali