Learning trajectory (LT) research in mathematics education has shaped both instructional materials and assessments. But, the body of LT research has also been critiqued for not adequately considering equity and addressing student diversity. This study begins to fill this gap by characterizing the reasoning of 23 multilingual students who participated in task-based interviews about proportional relationships and linear functions. Using tasks aligned with an established LT, the analysis focuses on the interaction of task language demand and student language background. Results show how task linguistic complexity can interfere with accurately interpreting the mathematical reasoning of emergent bilingual students. We discuss the need to (a) incorporate a focus on linguistic diversity when planning instruction and (b) broaden the students who participate in LT research to avoid reinforcing implicitly biased assumptions about diverse learners.