For linguistically diverse communities, researchers have long questioned assessment approaches that disregard learners' multilingualism. Our study offers potential approaches that center contextualized, multilingual knowledges and skills within classroom-based language assessments with one teacher and his English foreign language courses for pre-service English teachers in Oaxaca, Mexico. We combined critical ethnographic and participatory action research (PAR) methodologies to foster collaboration, which shaped the assessment design and implementation. In this article, we detail our methodological approaches and include views about the resulting assessment as reported by the pre-service teachers in post-assessment questionnaires and stimulated recall-interviews. This work points to the promise of adopting approaches that position researchers as individuals who promote and help advance the goals and values of participants. We discuss the implications of our approach not only in the creation of assessment instruments but how this collaboration also impacted teaching approaches and contributed to the laying of the foundation for sustaining and expanding the collaborative work with the teacher. We conclude with recommendations for making use of this contextualized collaborative approach in other settings.