Silent reading is the primary mode of reading for proficient readers, and therefore, silent reading fluency is often assessed in research and practice. However, little is known about the validity of the tests administered to students with different language backgrounds. Given that academic language is assumed to be especially challenging for students with a non-German home language, one might wonder if effects of academic language proficiencies can be found on these tests. In the present study, we explored whether, owing to academic language demands, students with a non-German home language (N = 748) would be found to be at a greater disadvantage than their monolingual-home peers (N = 1669) on the most frequently used silent reading fluency test in Germany. Using differential item functioning (DIF) analyses, we found specific item difficulties to the disadvantage of the students with a non-German home language. This DIF was linked to the academic language features of the sentences.