Lao, the official language
Lao, the country's official language, closely resembles Thai. To give you an idea, Lao's similarities with Thai are like those between Spanish and Portuguese. Unless you're fluent, there's not much use in relying on Thai to get by.
Learning Lao is no mean feat, so don't expect to be able to hold a conversation in the language. At the same time, it's good to learn the basics, which will help you meet people and navigate your way around. Knowing how to say 'hello', 'how are you?', 'thanks' as well as being able to count will help you get along.
As with Thai, the biggest challenge is being able to pronounce words. A slight change in intonation will completely alter the meaning of a word. But that shouldn't stop you from trying to speak the local language; Laotians will love that you're making the effort and you'll be warmly welcomed in return. No one's going to get upset if your pronunciation isn't perfect. If, when eating out, you manage to order in Lao, don't be surprised if you're met with astonishment or if you unleash fits of giggles.
Speak in English.
The few words that you'll pick up in Lao won't be enough to hold down a conversation. Fortunately you can fall back on speaking English. If English doesn't work and if you know some French, you can always try the latter during your trip to Laos , particularly when speaking to the older generation. You'll also see that official documents are written in both Lao and French. But encountering French speaking Laotians is becoming rare. Young Laotians tend to speak English and it's more widely spoken than its European counterpart. Throughout the country you'll find that anyone in contact with tourists will be able to speak English.