Language use is central for accessing, learning, and communicating understanding of undergraduate engineering knowledge. There is a need to understand students' experiences, interpretations, and expectations of language use -- conceptualised as related notions of language and disciplinary literacies -- in their transition from school to and through first-year university. We use interviews with nine first-year mechanical engineering students to describe this transition as a to-and-fro process of navigating a continuum of complexity. All students experienced this as a process of uncritical socialisation into undergraduate engineering. Yet navigating this continuum was harder for some, a process that was inflected through their language repertoires and schooling experiences, but also agentive and personal. Students identified an absence of opportunities to bring their knowledges, languages and identities to this process. These findings and our conceptual tools have purchase in wider debates concerned with the transition into engineering programmes in which diversity is increasingly the norm.