3.3 Priority document 

Where the certified copy of the previous application whose priority is claimed (priority document) is not in an official EPO language, a translation into one such language need only be filed at the invitation of the EPO. This invitation is issued only where the validity of the priority claim is relevant to determining the patentability of the invention concerned. The translation may be replaced by a declaration that the European patent application is a complete translation of the previous invention.

See A‑III, 6.8 for more information on the translation of priority documents.