An official language isn't a mandate that enforces the use of English (take as an example India, which is probably the most multi-lingual country there is, but has Hindi as the "official language of the government". It's not the "national language", though in their case the splitting of hairs is probably directly tied to the number of languages spoken).
I know an American who spent several years in Paris, working in English, and never learned more French than was necessary to order her coffee. And this is despite French being the national language of France. (Interestingly enough, France is very multi-lingual as well, with many regions having their own history and language like Provençal, or Niçois, Breton, etc.).
It's more about a recognition of the status quo (as another commenter pointed out, most people acquiring US citizenship are required to demonstrate the most basic level of proficiency with the english language), and ensuring a baseline of support, no matter where you are in the country. Multi-lingualism is a plus, and in no way hindered by having a national language