Talk
While the national language Thai is used in schools and thus well-understood, the local Isaan language, which is a dialect of Lao, is predominant. Khmer is also widely spoken in areas near the Cambodian border. Although the person you meet in the market might speak little or no English, it's more likely than not that they are already bilingual or multilingual.
Understand
Isaan is a wonderful part of Thailand to visit if you became tired of Khao San Road, one temple after another, or the tropical beaches. The relative isolation and underdevelopment of the area means that Isaan is a good place to get a little off the beaten track and discover Thailand's agricultural roots and natural scenery.