In the early days of the EU’s existence (when it was known as the EEC), French was the dominant language. Still recognised as the language of diplomacy, French was spoken in three of the six original Member States including the only large Member State to have emerged from the second world war with its moral authority more or less intact. The accession of the UK and Ireland in 1973 brought into the EEC the world’s nascent world language, English.
Initially French remained widely used but the growing number of Nordic Member States and the Great Enlargement to the east and south in 2004 and 2007 led to a significant growth in the use of English, the most widely spoken second language. It is only the Court of Justice of the European Union, based in Luxembourg and heavily influenced by French law and legal thinking, where French retains its erstwhile pre-eminence. Perhaps Monsieur Barnier sees Brexit as an opportunity for one of the EU’s former imperial powers to strike back.
The EU Treaties do not say anything about the language in which withdrawal negotiations are to be conducted.
However, in the first article of the first regulation adopted by the EEC back in 1958, the official languages and the working languages of the institutions were listed. The list has been kept up to date with each successive enlargement and now contains 24 languages, including English (and French). The Treaties themselves are authentic in all 24. Citizens of the EU (in other words, nationals of the Member States) are entitled ‘to address the institutions and advisory bodies of the Union in any of the Treaty languages and to obtain a reply in the same language.’
As a result, the institutions are serviced by armies of translators (who translate written documents) and interpreters (who translate speech). They sometimes use English as a bridge between other less widely used languages. So it was doubtless assumed by the author of Article 50 (a British official) that, in the unlikely event that it was ever invoked, the normal practices of translation and simultaneous interpretation would be used.
There are practical reasons why this is likely to be the outcome. While the Commission officials who conduct the negotiations with the UK would have no difficulty using French, national ministers who, as members of the Council, will supervise the Commission and conclude the agreement on behalf of the EU, may well wish to use their own languages.
They will of course include the Prime Ministers of Ireland and Malta, who have been prominent contributors to the Brexit debate. In both countries, English is an official language which they may prefer to Irish or Maltese. European Parliamentarians will expect to be able to use any official language when considering whether to consent to the agreement, as required by Article 50. Many will choose English. So while Monsieur Barnier doubtless enjoyed the outrage his suggestion provoked on the other side of the channel, it is no more than a tempête dans un verre d’eau (as the French say). He may find that English remains an official language even after the Brits have left.