Any party to oral proceedings before the EPO may, in lieu of the language of the proceedings, use one of the other official languages of the EPO, on condition that such party either gives notice to the EPO at least one month before the date laid down for such oral proceedings or makes provision for interpreting into the language of the proceedings. In the former case, it is the responsibility of the EPO to provide for interpretation at its own expense.
A party must be clear as to which official language it wishes to use. It then has a right to both speak and hear that language, as long as the conditions of Rule 4 have been fulfilled. The party does not, however, have a right to have one language in which it will speak and a different language in which it will hear (see T 774/05).
The language of the proceedings as defined in Art. 14(3) cannot be changed. This means that any amendments to the application or patent have to be filed in the language of the proceedings (Rule 3(2)).
If all parties have indicated that they will use another official language, the division may depart from the language of the proceedings so as to manage without or with fewer interpreters (this question normally arises only in opposition proceedings). The parties' summonses are therefore accompanied by information which encourages them to agree how this can be achieved.
It may be possible to agree to limit the interpreting to "one-way", i.e. from one language into another but not the other way round. If a comment made in one language has clearly been misunderstood, the division may clarify it in another. Under no circumstances however can its members officially act as interpreters.