Traumatic brain injury (TBI) often results in cognitive impairments that require investigators to consider language accessibility of survey instruments, clinical evaluations, and other research tools. We describe an iterative language validation process for the Behavioral Assessment Screening Tool (BAST) and BAST Spanish version (BAST-ESP), consisting of two phases: (1) achieving an accessible literacy level for English-speaking people with TBI and (2) translating, validating, and cognitively testing the BAST-ESP for Spanish-speaking people with TBI. Investigators recruited scientific experts and members of the target populations to adapt and test the surveys. Modifications to original survey instruments included simplified semantic structures, enhanced conceptual clarity, rephrased idiomatic expressions, and rewording to bridge cultural differences in linguistic connotation. Findings from participants in focus groups and cognitive interviews confirmed accuracy and ease of comprehension and informed further adjustments and content relevant to the specific target populations. We demonstrate the importance of a systematic adaptation and validation process to develop a lower-literacy instrument appropriate for people with cognitive deficits and to enhance the BAST-ESP beyond translation alone. This article, along with a previously published article about BAST content validity process, provides a road map for other investigators to conduct systematic adaptation of scientific instruments for low-literacy and non-English-speaking populations. [HLRP: Health Literacy Research and Practice. 2019;3(4):e243-e249.].