This is the 10720th most frequent Russian word.
восточноевропейские
Eastern European.
Here, 'восточноевропейские' is an adjective describing 'страны' (countries). It specifies a geographical region in Europe.
Восточноевропейские страны имеют богатую культурную историю.
Eastern European countries have a rich cultural history.
In this sentence, 'восточноевропейских' is an adjective qualifying 'языках' (languages), indicating languages from the Eastern European region.
На восточноевропейских языках говорят миллионы людей.
Millions of people speak Eastern European languages.
Here, 'восточноевропейская' modifies 'архитектура' (architecture), attributing the quality or style typical of Eastern Europe.