Lost in the Mix: Evaluating LLM Understanding of Code-Switched Text
Authors: name: Amr Mohamed, name: Yang Zhang, name: Michalis Vazirgiannis, name: Guokan Shang

Code-switching (CSW) is the act of alternating between two or more languages
within a single discourse. This phenomenon is widespread in multilingual
communities, and increasingly prevalent in online content, where users
naturally mix languages in everyday communication. As a result, Large Language
Models (LLMs), now central to content processing and generation, are frequently
exposed to code-switched inputs. Given their widespread use, it is crucial to
understand how LLMs process and reason about such mixed-language text. This
paper presents a systematic evaluation of LLM comprehension under
code-switching by generating CSW variants of established reasoning and
comprehension benchmarks. While degradation is evident when foreign tokens
disrupt English textx2013even under linguistic
constraintsx2013embedding English into other languages often
improves comprehension. Though prompting yields mixed results, fine-tuning
offers a more stable path to degradation mitigation.