1900s/1934: The Chocolate Box
The separation of church and state debate is an important issue, plus Poirot mentions it in Peril at End House (the book, chapter 15). In the adaption, the “language law” is mentioned (possibly the Law on Equality, 1898?). Paul Deroulard tells his wife that "we're into a new century, but you are stuck in the last!".
The year 1934 is chosen because Poirot says “20 years and you look the same” when meeting an old friend. Assuming he left Belgium in 1914, 20 years would be 1934. (The actual case of the chocolate box is also stated as taking place “just before the war”). Finally, in Cards on the Table, Poirot tells a suspect that his last mistake was 28 years ago. As you will see, Cards on the Table probably takes place in 1937. Assuming that The Chocolate Box was his last mistake, this means that the story would have to take place in 1909. How Poirot would have had time to become head of the Belgian police by 1914 (see timeline) if he was "a young police officer" in this episode, is another question. I haven't quite decided myself yet, but in my mind it would sound better if The Chocolate Box was set at the turn of the century (ca. 1900). Furthermore, I think it's debateable to what extent Poirot made a "mistake" in this case, so "28 years ago" may just as easily refer to another case.