In written proceedings before the EPO any party may use any official language of the EPO. However, in such proceedings the EPO uses the language of the proceedings in the sense of Art. 14(3). In written proceedings the organs of the EPO cannot use any official language other than the language of the proceedings (see G 4/08).
Art. 14(3) and Art. 14(4)
Natural or legal persons having their residence or principal place of business within an EPC Contracting State having a language other than English, French or German as an official language, and nationals of that state resident abroad, may file documents which have to be filed within a time limit in an official language of that state (“admissible non-EPO language”). For example, an Italian or Swiss applicant may file a reply to a communication from the Examining Division issued under Art. 94(3) in the Italian language. A translation of this document, into an official language of the EPO must be filed (Rule 6(2)). The translation can be into any EPO official language, regardless of the language of the proceedings.
Normally, the period allowed for filing this translation is one month after filing of the document, but if the document is a notice of opposition or appeal, or a petition for review (Art. 112a), the period extends to the end of the opposition or appeal period or the period for the petition for review, if this period expires later.