Fascinating, and prompted me to find the extensive discussion of Swahili at
http://victorian.fortunecity.com/louvre/88/swahili/swahili_history.html, and the continuation link at the bottom of that page. Extracts:

The Swahili language, is basically of Bantu (African) origin but with strong Arab and Persian influence. There are those that assert that the language came about as a result of Arabs and Persians coming to the East African coast and those that maintain that the language was spoken in East Africa before the coming of Arabs and Persians.
...
Such borrowing is comparable to the proportion of French, Latin, and Greek loans used in English. Although this proportion for Arabic loans may be as high as 50 percent in classical Swahili poetry (traditionally written in Arabic script), it amounts to less than twenty percent of the lexicon of the spoken language.