Cope
talk
Being a part of Indonesia, all locals will be able to speak Bahasa Indonesia. However, the ethnic Chinese can often be heard speaking a localised variant of Hokkien known as Medan Hokkien, which is almost identical to Penang Hokkien spoken across the Strait of Malacca.
Medan has a very high level of education and many younger people speak English very well. I have found quite a few older people, those over 50 or 60 also speak English and some even Dutch. If you find yourself stuck just tell the person you need someone who speaks English to help and there will be someone there in a matter of minutes. Speaking English well is a highly regarded skill by the locals so anyone who does so is well known and can be found quickly.
Bahasa Indonesia is also a simple language to learn so get yourself a phrase book and learn a few simple phrases, you will feel more clever as well and the more you try to speak it the quicker you will pick up the basics, its easy and fun and the locals will really appreciate your efforts although they will laugh loudly with you too.