To ensure fairness, it is important to better understand the relationship of language proficiency to standard psychometric analysis procedures. This paper examines how results of differential item functioning (DIF) analysis are affected by an increase in the proportion of examinees who report that English is not their first language in the analysis sample of a large-scale assessment. The results vary by group. In some combinations of focal/reference groups, the magnitude of DIF is not appreciably affected by whether the DIF is performed on examinees whose first language is not English. In other groups, the first language status matters. The results vary by type of test as well. In addition, the magnitude of DIF for some items is substantially affected by whether the DIF is performed on examinees whose first language is not English. One appendix is included containing 30 figures with results. (Data charts are appended. Contains 9 tables, 42 figures and 4 notes.)