Based on long-term ethnographic fieldwork among Esperanto speakers and Esperantists - mostly, but not only, in Paris - this thesis asks: how can the Esperanto community and movement come into being in practice if Esperanto speakers are scattered all over the world and may, at times, not share much beyond the language? Created in the late nineteenth century and alternatively supported by intellectuals and left-wing activists since its early days, Esperanto has currently also attracted the interest of young people through its online use. Coming to be more than a language, it has developed a widespread, geographically scattered speech community, a language-based social movement, and a set of cosmopolitan principles and sociabilities linked to it. I aim to contribute to debates about cosmopolitanism, globalisation, international communication, mobilities, and hopes for the future, as well as political, digital and language activism, by analysing perspectives of Esperanto speakers and Esperantists. Addressing the contrasting ideas that posit Esperanto as 'a thing of the past' and 'the language of the future', I look at the ways the language has been used in the present, through everyday practices, to mediate between people from different national backgrounds. France, particularly Paris, provides a setting in which my research questions resonate with national debates on politics and languages. This creates a fruitful environment to analyse how Esperanto is frequently seen through the lenses of either engagements with traditional social movements (such as communism, anarchism and pacifism) or as an intellectual game and tool to build sociability networks that extrapolate French territory. From these issues, my main argument is that the ephemeral nature of the enactments of the Esperanto community is what makes engagements with this language particularly appealing and productive for its speakers and supporters.