Mobilisations in France for the rights of foreign nationals are sometimes the setting for language practices based on a language ideology that characterises the political voice as a possible path to empowerment. This article examines one practice in particular, labelled here as ‘don’t speak on their behalf’, as it manifests in this specific battleground. The research is situated at the interchange between discourse analysis and sociolinguistics. The findings show that ‘don’t speak on their behalf’ was not a simple moral requirement but rather a politicised practice that took on meaning for the activists in relation to what they identified as the dominant groups’ appropriation or invisibilisation of the minority groups’ voices in the public space. The following hypothesis was laid: language is, for those activists, a resource to express their solidarity towards undocumented foreign nationals. This language ethics practice manifested at different linguistic levels and permeated interactions, discourses and actions. It is conceptualised here as a register (Agha 2007) because it was found to be a pervasive interactional norm that was characterised by its multimodality. Its various manifestations are illustrated at the mimetic/postural, pragmatic and interactional levels.