The official language of the Commonwealth is English. The Commonwealth is an association of countries that were once part of the British Empire, and English was the language of the British colonizers who ruled these countries. Today, many of the countries in the Commonwealth continue to use English as their official language, even though some may also have other official languages, such as French, Spanish, or Dutch. However, it's important to note that not all Commonwealth countries have English as their official language, and some may have more than one official language.