SQ makes announcements in all four-Malay, English, Chinese/Mandarin/Hokkien, and Tamil?
Whether English is lingua franca or not is irrelevant. In absence of a common language, English is the default language of communication. I suppose the airline is obliged to not prefer one over others. Are all Government documents and communications conducted in all four languages?
As I understand, the term official language does not mean that every communication must be in all four languages but that, one must communicate in one of these languages to establish that you did communicate in the legal sense. There are more than a dozen languages, which have the status of an official in one or more states. It does not mean you have to communicate in all of those languages. The currency does have the amount written in all those languages.