Multi-lingual software systems build on interconnected components that are implemented in different programming languages. The multi-lingual nature of such systems causes additional complexity, for instance, when developers aim to identify what components of a system use the same data. Organizations and developers typically aim to adhere to a specified system architecture to avoid certain dependencies between multi-lingual components. However, such dependencies may still be introduced and only resolved later on. Thus, we refer to them as unintended dependencies: dependencies that may exist, but are not wanted by the developers or organization. There has been little research on multi-lingual systems so far, and dependencies within such systems have not been studied explicitly. With this paper, we tackle this issue by contributing a catalog of unintended software dependencies in multi-lingual systems. We elicited it by interviewing 17 practitioners at ASML. We report eight types of unintended dependencies, their causes, the resulting problems, and how they can be resolved. Further, we connect our findings to research on software smells and dependencies in monolingual systems. Our contributions serve as recommendations for practitioners on how to deal with unintended dependencies, as supportive evidence for existing research, and as basis for new techniques for managing dependencies in (multi-lingual) systems.