The construction and effects on national boundaries have become central topics in public and academic debates on digital sovereignty. Both state and non-state actors increasingly consider jurisdictions and traditional governing structures as means to capture and regulate digital data flows. This article delves into the intricate phenomenon of ‘data localization’, conceptualizing it as a socio-technical assemblage reflecting the evolving expectations surrounding Internet architecture and national boundaries. Interviewing the users of Threema – a Swiss secure messaging app – this study unravels data localization practices as a hybrid black box, intertwining technical changes, political discourses, socio-technical imaginaries, and shifting social norms. Drawing on the field of Science and Technology Studies, we mobilize the analytical tools of controversy and discourse to highlight data localization as a locus of political contestation in Switzerland, where imaginaries of national boundaries are often mobilized to symbolize security and reliability. The article provides three key contributions to the discourse on digital sovereignty, fragmentation, and governance. Firstly, it argues for the usefulness of Science and Technology Studies in understanding Internet governance, emphasizing the need for analyses grounded in specific socio-technical contexts. Secondly, it advocates for a social perspective on digital sovereignty, emphasizing user agency, social movements, and collective action as crucial factors shaping the governance of data flows. Lastly, the article sheds light on users resorting to state jurisdictions as a means to reinforce control over data flows, exploring the discursive mobilization of national boundaries in the digital public sphere.