Assessing cognitive demand is crucial for research on self-regulated learning; however, discrepancies in translating essential concepts across languages can hinder the comparison of research findings. Different languages often emphasize various components and interpret certain constructs differently. This paper aims to develop a translingual set of items distinguishing between intentionally invested mental effort and passively perceived mental load as key differentiations of cognitive demand in a broad range of learning situations, as they occur in self-regulated learning. Using a mixed-methods approach, we evaluated the content, criterion, convergent, and incremental validity of this scale in different languages. To establish content validity, we conducted qualitative interviews with bilingual participants who discussed their understanding of mental effort and load. These participants translated and back-translated established and new items from the cognitive-demand literature into English, Dutch, Spanish, German, Chinese, and French. To establish criterion validity, we conducted preregistered experiments using the English, Chinese, and German versions of the scale. Within those experiments, we validated the translated items using established demand manipulations from the cognitive load literature with first-language participants. In a within-subjects design with eight measurements (N = 131), we demonstrated the scale’s criterion validity by showing sensitivity to differences in task complexity, extraneous load manipulation, and motivation for complex tasks. We found evidence for convergent and incremental validity shown by medium-size correlations with established cognitive load measures. We offer a set of translated and validated items as a common foundation for translingual research. As best practice, we recommend four items within a reference point evaluation.