что следовали (...that followed)
Russian foreign policy had been undergoing important changes in the decades that followed the Congress of Berlin.
Русская иностранная политика испытывала важных переменые при десятилеиях что следовали Берлин Конгресса.
политика - is feminine; hence, Russian (adjective) and foreign (adjective) are feminine adjectives
испытывала - had been undergoing (past imperfective)
in the decades that followed the Congress of Berlin is PREPOSITIONAL ; hence, declension follows prepositional
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