In my opinion that's best to be done to visit a certain country and stay there for a few weeks. Due to work, family et cetera, that's not always possible, so I refresh it by looking at certain TV series. "Les Petits Meurtres d'Agatha Christie" I just stumbled on and I like it very much: the era, the characters, it's "crime" and Marlène of course. For my English for example I recently watched "Life on Mars".