At the age of 34 (1980) I had to learn Dutch and to work in the language. I was monolingual prior to that date. It was the most difficult but most rewarding thing that I ever did. Here are the key factors:
▪️From a certain date my Dutch colleagues (all fluent in English) only spoke to me in Dutch.
▪️I lived not in an expatriate enclave but in a 100% Dutch community. Neighbours helped enormously.
▪️I took an interest in Dutch life – politics, culture, cuisine, festivals.
▪️I watched Dutch television and read a Dutch newspaper.
▪️I had a charming teacher who took me through the language not with text books but with chats and anecdotes.
Ten years ago as a Gamesmaker at London2012 I was assigned to the Dutch Olympic team. Despite the long gap I still (just) managed with the language. 😀