Arabic is the official language in Qatar, although it is safe to say that the majority of the population does not speak it. The number of foreign workers from the Indian subcontinent and the Philippines far exceeds the number of local Qataris, especially in Doha (where foreigners make up about 90 per cent), many of whom have very limited knowledge of Arabic.
English is the lingua franca, and many Qataris speak it to communicate with foreign workers who work for them. Because Qatar was a British protectorate and because of the high level of globalisation, English is the second most widely spoken language and most residents will be able to converse in at least basic English. The Arabic dialect spoken among local Qataris is the Gulf dialect. Some foreign workers may not understand English or Arabic and rely only on their mother tongue. You may encounter foreign workers who speak different languages such as Afrikaans, Mandarin, Japanese, Hindi, Urdu, Tagalog and Thai.
Although you will do very well in Qatar if you speak only English, your hosts and any other locals you meet will be very impressed and grateful if you can recite a few basic phrases in Arabic.