The London Agreement is a treaty under which the signatory countries agree to waive, for the most part, the requirement that patents be translated into the national language of each European Patent Convention country. Previously, when a patent application was granted by the European Patent Office (the EPO has always permitted patent filings and correspondence to be carried out in either English, German or French), most European Patent Convention member states required the filing of a complete translation of the patent specification into the national language of that state in order for the patent to take effect there. Consequently, translation costs accounted for as much as forty percent of the total cost of securing European patent rights.
Last October, the French Parliament finally ratified the London Agreement after initially refusing to do so. The London Agreement consequently went into force on May 1, 2008 and applies to patent applications granted after that date. Post-grant translation requirements have been relaxed considerably, and the advantage is maximized when the original filing was conducted in English. Further details may be gleaned at here.