Few mobile-assisted vocabulary learning approaches fully correspond to the authenticity principle of the situated learning theory (SLT). A novel practice of changing the smartphone system language to second language (L2), which intends to better implement SLT in a real smartphone context, is the focus of this study. To explore the potential of using this under-researched approach to facilitate L2 vocabulary learning, a mixed-method design was adopted to investigate its impact on vocabulary acquisition, global language learning motivation, and vocabulary learning motivation. Participants’ pre- and post-test scores on the vocabulary knowledge test and motivation questionnaires were compared between the experimental and control groups. Interviews were also conducted to understand learners’ perceptions of this novel approach. Both quantitative and qualitative results indicated that the approach of changing smartphone system language to L2 was effective in increasing learners’ vocabulary knowledge gains and improving their global and vocabulary learning motivation. These findings contribute to the literature on L2 vocabulary learning and provide significant pedagogical implications.