India is a linguistically diverse country with 18 different languages and 1,600 dialects. Hindi is the national language of India. In Maharashtra, the main language is Marathi, which is commonly spoken in Mumbai. But because it is a melting pot of many cultures, Mumbai has its own language, a unique blend of Gujarati, Marathi, Hindi, and English called "Bambaiyya Hindi."
The good news for English speakers is that English is fairly common in Mumbai, so you don't need to learn Bambaiyya Hindi to survive. Most shop signs, traffic signals and many advertisements around the city are written in both Hindi and in English. Bookstores sell mainly English titles. English-speaking nannies, household help and drivers are relatively easy to find. Many people speak at least some English, though statistics say that only 3% of Indians are proficient. It helps to learn a few Hindi phrases for bargaining or just to be friendly, but in hotels, in the business district, and in areas thick with tourists, you can get by on English alone.
The prevalence of English can be comforting for English-speaking expatriates. However, the English spoken in India is not your standard English; it is a special variety called "Indian English," spoken with a distinctive Indian flavor. It has a sing-song ring to it. You will quickly notice that the word "only" is overused and that the definite article is often dropped. Indians speak English quickly without Western inflection, which can make it hard to understand. With time, you'll develop an ear for it and might even start incorporating some of the quirky aspects into your own speech.
A few common examples of Indian English you will come across in Mumbai: