An apostille is a government-issued certificate that authenticates the origin of various public documents used in international dealings. Countries that are party to the 1961 Hague Convention require apostilles as a means of expediting the verification and legalization of documents from abroad.
Per the rules of the Hague Conference on Private International Law, an apostille is technically valid no matter the language in which it is drawn up. In practice, however, this rule is often not implemented correctly, or is overridden by local regulations that require all documents to have certified translations. This rule also does not apply to any other documents covered by the apostille.
Rather than face the lengthy bureaucratic process of contesting these requirements, most parties prefer to have the apostille translated alongside other pertinent documents.