Consulting the best available evidence is key to successful conservation decision-making. While much scientific evidence on conservation continues to be published in non-English languages, a poor understanding of how non-English-language science contributes to conservation decision-making is causing global assessments and studies to practically ignore non-English-language literature. By investigating the use of scientific literature in biodiversity assessment reports across 37 countries/territories, we have uncovered the established role of non-English-language literature as a major source of information locally. On average, non-English-language literature constituted 65% of the references cited, and these were recognized as relevant knowledge sources by 75% of report authors. This means that by ignoring non-English-language science, international assessments may overlook important information on local and/or regional biodiversity. Furthermore, a quarter of the authors acknowledged the struggles of understanding English-language literature. This points to the need to aid the use of English-language literature in domestic decision-making, for example, by providing non-English-language abstracts or improving and/or implementing machine translation. (This abstract is also avaialble in 21 other languages in Supplementary Data 4).