You may encounter a bit of problem communicating with street food vendors and with drivers of vans and rickshaws. In this situation, just request for help of anybody around you, he will be more than happy to interpret for you. English is second language in Pakistan and is widely spoken in the cities only.
The Languages spoken in Lahore are actually quite complex. What I mean is that there seems to be a seperate language spoken in Lahore which many refer to as "Lahori", which is an amalgam of Urdu, English and Punjabi. So I would say that some knowledge of all three is essential if you want to travel the city and talk to people from all walks of life. However, English will suffice if you simply plan to stay for buisness, and Urdu should be sufficient if you only plan to shop or take care of buisness.
Here are few sentences which might help you:
1. Asalam-u-aleikum = Hello
2. Waleikum Asalam = When someone says Hello to you.
3. Shukeria = Thanks
4. Mera nam Umer hay = My name is Umer.
5. Appka Naam kiya hay = What is your name ?
6. Mujhe Chai chahiye = I want Tea.
7. Ghoo-salkhana kahan hay = Where is the Toilet?
8. Airport kahan hay = Where is Airport?
9. kitne paese ? = How much ?
10. Allah hafiz = Good Bye