Third-party observations (E‑VI, 3) must be filed in writing and in one of the EPO's official languages. Supporting documents, e.g. prior-art citations, can be written in any language.
If the third-party observations and/or prior art are not in an official language of the EPO (Art. 14(1) EPC), the EPO may invite the third party, if identifiable, to submit a translation of the observations and, where appropriate, of the cited prior art in an official language within a period according to Rule 132.