Which Religions Cross Languages? Translation Permeability in a Four-Language Web Corpus on Religion and Mythology

Multilingual reference websites are usually described by how much they cover. This paper measures what crosses between their languages. Using a four-language (Turkish, English, Spanish, French) corpus of 7,016 articles covering 3,656 distinct subjects published 2003–2026, it introduces translation permeability: the share of subjects originating in one language that acquire a counterpart in another. Findings. (1) Parallelism is the exception — 60.3% of subjects exist in a single language, only 17.5% in all four, and the distribution is bimodal: translation is close to an all-or-nothing editorial decision. (2) Permeability varies more than sevenfold across traditions, from 95% (Mesopotamian mythology) to 18% (Judaism) and 13% (science and religion); culturally distant subjects travel, while subjects entangled with the local context stay monolingual. Islam, the largest category at 836 concepts, is translated at only 29% — depth of coverage and willingness to translate move in opposite directions. (3) The pattern is procedural, not intrinsic: when production switched to simultaneous four-language publication in 2026, permeability moved from 39.4% to 100%. Permeability is proposed as a diagnostic for multilingual reference corpora generally, because it exposes editorial selection that aggregate coverage counts conceal. The dataset is openly licensed and the analysis fully reproducible.