This study examines the impact of the linguistic transition from Arabic to French on the comprehension of mathematical concepts among first-year middle school students in Morocco. Utilizing a mixed-methods approach, the research analyzes student performance in identifying quadrilaterals and calculating their area and perimeter in both languages. Results indicate a significant decline in students' mathematical performance when instructed in French compared to Arabic. The study also explores the perspectives of 249 mathematics teachers, revealing a predominant preference for Arabic as the language of instruction. The findings suggest that the sudden shift to French imposes cognitive challenges, potentially hindering students' academic progress. This research contributes to the ongoing debate on the efficacy of bilingual education in Morocco and highlights the need for gradual implementation and enhanced language support to optimize learning outcomes.