With the benefit of hindsight it is comparatively easy to underestimate the obstacles presented by the idea of British military intervention on the continent in the case of attack on France by Germany. These obstacles are both mental – centuries of enmity and more recent colonial rivalry had hardly been attenuated by the limited scope of the 1904 Entente Cordiale – and also practical for a coalition which had last fought together against a non-colonial opponent in the Crimea half a century earlier. Much had changed since then, both armies had been the object of considerable reforms in order to professionalise them. Furthermore French no longer enjoyed the unrivaled status as universal language of exchange between educated Europeans. By investigating first the French and British structures in place to handle languages over the course of the 19th century and then the joint plans established from 1905 onwards, we intend to show to what extent the very different recent military experience for the British and the French led them to approach what would be the First World War in subtly different ways as far as their coalition functioning on French territory was concerned.