The country’s language watchdog is investigating after a Dutch-speaking commuter protested a conductor’s use of “bonjour” – French for “good morning” – to welcome him onboard during a rush-hour train from Mechelen, in Flanders, to the capital, Brussels, in October.

Writing on Facebook, Ilyass Alba, the French-speaking conductor, said that on the day in question he greeted passengers entering his carriage with a resounding “goeiemorgen, bonjour”.

The use of both the Dutch and French greetings was not good enough for one Dutch-speaking passenger, who told him off, saying: “We’re not in Brussels yet, you have to use Dutch only!”

The passenger was technically right, as under Belgium’s complex language rules conductors should in theory use both languages only in Brussels and a few other bilingual regions.