Translation
When a historic document survives only in translation, the original having been lost, researchers sometimes undertake back-translation in an effort to reconstruct the original text. An example involves the novel The Saragossa Manuscript by the Polish aristocrat Jan Potocki (1761–1815), who wrote the novel in French and anonymously published fragments in 1804 and 1813–14. Portions of the original French-language manuscript were subsequently lost; however, the missing fragments survived in a Polish translation that was made by Edmund Chojecki in 1847 from a complete French copy, now lost. French-language versions of the complete Saragossa Manuscript have since been produced, based on extant French-language fragments and on French-language versions that have been back-translated from Chojecki’s Polish version.

Q: What do researchers use back-translation to try to reconstruct when a document survives only in translation?
the original text