Swahili and English are the official languages of Tanzania. Swahili however is the dominant language of society, with English largely limited to commerce and higher education. Over 100 different languages are also spoken by individual ethnic groups, though Swahili is almost universally spoken across the country.
Time of Day
This is where a little knowledge of Kiswahili can cause some inconveniences. Tanzanians don't function on the same time as Westerners. This doesn't mean Africa time, which is the notion that appointments are flexible and people can arrive when they please. For Tanzanians, it's illogical that the day would start in the middle of the night.
Since sunrise and sunset happen pretty much at the same time all year round, 6AM and 6PM, the day starts at 6AM which is 0 hours. So when telling time in Kiswahili, Tanzanians always subtracted 6 hours for western time. 11AM is 5a.m to a Tanzanian. To avoid any confusion, a Tanzanian will tell time in English if they want to use the western standard and in Kiswahili if they use local standard.
If you want to practice your Kiswahili, just keep this in mind if you discuss appointment times with a Tanzanian. If you say Saa kumi na moja asubuhi(11AM), instead of Saa tano asubuhi (5AM), you'll end up waiting for 6 h if the person arrives on time, plus however long it takes to arrive fashionably late!