Neither
Well, South Africa actually has 3 capitals: Pretoria is the Administrative Capital, where the civil service and most embassies are located.
Cape Town is the Legislative Capital - parliament is there
Bloemfontein is the Judicial Capital - the supreme court of appeals is there
Johannesburg is not a capital at all - is is however the economic centre of the country.
Johannesburg was not changed to jozi, Jozi is a nickname for Johannesburg. The official name is still Johannesburg, commonly people (mainly locals) refer to it by nicknames; Jozi or Joburg.
Johannesburg does not have a new name, it is still Johannesburg.
Swahili isn't an official language in Johannesburg, but it would be spoken by Kenyan people living in Johannesburg.
Johannesburg is a city in South Africa, so the official languages are English and Afrikaans.
That would be impossible to say as there are millions of informal houses in and around Johannesburg which would not form part of an official count.
Johannesburg
You can find out where to reach Jehovah's Witnesses in most countries on their official website at jw.org. There are seven Kingdom Halls in Johannesburg, South Africa. See related links for location
There are 12 official languages in South Africa, but a lot more are spoken due to people coming from all over the world. This is the same as Johannesburg, as the most populated city in the country. However the main and most common languages spoken in Johannesburg are English, Zulu & Tshwana.
OR Tambo International Airport.
Soweto
Johannesburg
Soccer City Stadium.