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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