This paper will describe the development of UniOn!, an application for mobile devices, produced within the ILOCALAPP project between October 2015 and August 2018, and launched thereafter in its finalised version. Even though the use of mobile technology for learning purposes is increasingly taken for granted in many contexts, it still represents a great challenge in terms of (re)conceiving education. As Traxler & Kukulska-Hulme (2016) point out, the context for mobile learning has changed and become more and more context-aware, due to the ubiquity of the social use of mobile devices. These topics were exploited during the ILOCALAPP project, and its main result the UniOn! application for language and culture learning represents a case study in this regard. We will begin this contribution with a description of the project and the application, before discussing in detail the design considerations. We will conclude by presenting the pilot phase during which the app was tested and revised before its final release. Throughout this paper, special attention will be paid to the collaborative approach, which characterised the design, the development and the testing of the app, and which led to the features of UniOn!’s current version.