One djinn, two djinni. Or, in Latin, one genius, two genii. Which devolved to "I Dream of Jeannie".
Yes, with one exception.
In Hebrew, initialisms or abbreviations of one word are one letter and an apostrophe. Examples:
The first is ר׳ which is usually short for rabbi. The second is ג׳ which is usually short for גברת (g'veret), meaning Ms.
So it's always a J sound unless the entire word is just one letter and an apostrophe.