Watch for allusions and hidden meanings, and listen to what is not said as well as what is said. Understatement is usually the rule. East Europeans consider it impolite to say no directly. When using an interpreter, speak slowly, avoid complicated words, repeat important ideas in a slightly different fashion to make sure you are getting through, and look at the person being addressed, not the interpreter. English, the language of choice, is becoming the lingua franca of Eastern Europe, the language Balts use to talk with Bosnians.
Older people are more likely to know German or French.
And all of them will have studied Russian, although in most countries they are reluctant to speak it. Don't drink the water. Bottled water and soft drinks, both domestic and imported, are available in hotels and stores. Tap water should be boiled. Hospitality Plus Hospitality in Eastern Europe is legendary.