Mambo, Sergio? Sounds like a fantastic trip, and your Swahili is really good! I'm impressed that you're using the relative infix in your writing (waliOzungumza)--that was really hard for me to understand. Safi sana. I am not a native Swahili speaker, but I am at an advanced level and have been studying for years. I'll correct where I see issues because it will hopefully be helpful for both of us. I hope that any native speakers that come along will call me out if any of my corrections are misleading!  Let me know if you have questions.

[1] "Kisiwa" is referring to a place, so it's in the locative class (I can't remember the class number). Places usually take the pa- prefix for adjectives, e.g., "Mahali hapa ni pazuri!"

[2] You're speaking in the past-tense (waLIozungumza), so you should be consistent. To say "there were no," in the past tense, you should say "hakukuwa na ____" because the first "ku" is to mark the negative past tense, and the second "ku" goes with the single syllable verb "wa." 

[3] You should say "there weren't any people," so it's plural "watu." To say "watu wowote" is to say "any people."

[4] I made this match the plural "watu" and added the plural relative infix "o" so that its "people which speak."

Kazi njema na safari njema!