Learn Sinhalese

It would be easy to travel in Sri Lanka speaking only English. (Your English can be very basic, but you should have some ability. Even in former European territories, like Pondicherry or Goa, very few people speak the colonial language.)

However, making an attempt, however limited, to get around in a local language can have a magical effect and can enhance your Sri Lanka experience. When you can say " No " or ask directions in a local language, you're demonstrating your interest in the local culture and implying local experience. This earns you respect and makes interactions easier. Of course, it also creates the assumption that you speak more of the language than you do.

The simple words suggested here, and the phrases you can build from them, are so basic that you'll be speaking only a "pigeon" form of the language. You will only be able to make a request or ask a simple question. You will not be able to interact with free speech. However, these words would get you started and you can easily add to your vocabulary as you go along. This is just to get you started.