From personal experience, I found that the two most useful phrases to learn are Thank You and I don't understand.

If you're travelling to Thailand, Thank You is pronounced phonetically as Cup Coon Krup if you are a male and Cup Coon Kha if you're a female. The second phrase is pronounced My Cow Jai. Use the first phrase often.

You may be surprised that one of the two is not Do you know English? If you ask if they know English in English, they'll reply in English. Otherwise, they'll just smile and say My Cow Jai.