The official language in which the application is filed or into which it is translated is the language of the proceedings in all proceedings before the Office, unless the Implementing Regulations provide otherwise (Art. 14(3) EPC).

In G 4/08 (OJ 2010, 572) the Enlarged Board of Appeal ruled that if an international application has been filed and published under the PCT in one official language of the EPO, it is not possible, on entry into the European phase, to file a translation of the application into one of the other two EPO official languages.