Under R. 114(1) EPC, any observations by a third party must be filed in writing in an official language of the EPO, i.e. English, French or German; otherwise, they are deemed not to have been received. Observations not filed in an official language and so deemed not to have been received are nevertheless added to the file and so accessible to the public under Art. 128(4) EPC. Third-party observations will be communicated to the applicant or patent proprietor even if they have not been filed in one of the prescribed official languages (Art. 14(4), R. 114(1) EPC; Guidelines A-VII, 4.2).
R. 114(1) EPC also explicitly provides for applicability of R. 3(3) EPC, which states that documentary evidence and, in particular, publications substantiating an argument may be filed in any language. The EPO may, however, require that a translation in one of its official languages be filed, within a period to be specified. If a translation is not filed in due time, the EPO may disregard the document in question.