Why is there an absolute must to only have one official language? Why not chose several most widely spoken ones, including English, and let each speaker chose the one that suits them best?
I live in a country where we have 4 national languages and 3 official ones. Our members of parliament can express themselves in the language of their choosing and their speech/intervention is translated for the rest. That way people can express themselves in the language they are most comfortable in (that is often not English, unless it's their mother tongue) and that does not put older generations (who never learnt English) at a disadvantage.
Having one single language in a multicultural setting can be tricky.