The official language of the EPO (English, French or German) in which the application is filed, or into which it is subsequently translated, constitutes the "language of the proceedings".
The language of the proceedings is the only language used by EPO departments in written proceedings on that application (G 4/08).
Where European patent applications are filed in one of the official languages of the EPO, or after they have been translated into one of them, the description, claims and drawings can only be amended in that official language, which is the language of the proceedings.
Any claims filed after the date of filing will need to be filed in the language of the proceedings.
Example: If an application is filed without claims in Japanese and is then translated into English, the claims will need to be filed in English. Subsequent amendments to the application will also have to the filed in English.