We examined whether English language proficiency (ELP) related to interim reading and math performance differently when academic assessments were administered to students in either English or Spanish. We also tested these effects with Spanish language proficiency (SLP) as well as jointly with ELP and SLP. Students from a midwestern suburban school district in Grades 2–8 were included (N = 2,327); 763 of these students in Grades 4–8 had SLP and ELP scores. Differences between English and Spanish math scores varied minimally across the majority of the ELP distribution. English–Spanish differences in reading across ELP scores were more pronounced. Language differences in math and reading scores as a function of only SLP were less clear, and reading was also more sensitive to joint changes in ELP and SLP compared to math. We discuss practical implications and limitations for assessment within multitiered systems of support as well as future directions for research.