English is the ONLY official language in Mauritius and is used for legal purposes. That however is not what is actually SPOKEN. On the coast at hotels and big tourist areas you will find English is widely spoken, along with French although both with an accent. The unofficial language if French and Creole with better educated people tending to use strict French and us common "Joes" speaking Creole which is a type of French (think of it like a French slang). There is also a large amount of immigrants from India and China who are not very adept at English, french, or Creole, but tend to learn Creole more as that is the widest spoken language in day to day situations such as at the market.
So, to give example, at beachfront hotel you can use any of the 3 French, English, or Creole, but the employees will not use Creole unless you do first.
If doing papers at government office they will all be in English, but again the clerk will use any of the 3 languages.
If shopping at a NON-tourist store Creole or French is spoken with some people knowing some English if needed.
At home around the dinner table it is Creole 90% of the time.
Generally people will speak french in formal conversation or if not a friend as Creole is sometimes considered rude - for example shopkeeper in a modern store would be considered rude to speak Creole to you if you had not spoken Creole to them first, then it is all ok and nothing rude about it.
English = legal language
French = formal language
Creole = informal / familiar language