I guess most, if not all, of classical-period works that have survived, were translated. But I'm certain many of the medieval era works were never published in a language other than their Latin original. Among those, I wonder which are the most prominent (at least at the time of their publish); namely those which had some noticeable impact.
One very important work seems to be never translated: The glossa ordinaria on the Corpus iuris civilis. This is the result of more than a century of work of law professors (esp. in Bolonga; caled glossators) commenting in glosses on the text of the (later so called) Corpus iuris civilis bundled by Accursius (d. 1263).
The glossa ordinaria became soon the the first resource when dealing with a question of Roman law. There was the saying: Quidquid non agnoscit glossa, non agnoscit curia. Until the early 17th century almost all printed editions of the Corpus iuris had also the glossa ordninaria.
To proof the absense of a translation is a bit difficult, but I think I can make it plausible.
I did not found any new print of the glossa ordinaria after 1627 (Lyon, digitalized, linked on Wikipedia). There is a reprint of a edition of Venice 1488 printed in 1968ff. (Corpus glossatorum iuris civilis. Curante iuris italici historiae instit. taurinensis univ. rectore ac moderatore Mario Viora. Augustae Taurinorum), but no critical edition (so explicitly Wikipedia; Brill's New Pauly (s.b.) does mention a repertorium of manuscripts, but no edition).
Seeing that there is no modern edition it is improbable that there is a modern translation. And in its best times everyone dealing professional with Roman law knew enough Latin to use the glossa ordninaria, so an old translation is improbable too.
Dolezalek, Gero (Aberdeen RWG), “Glossators”, in: Brill’s New Pauly, Antiquity volumes edited by: Hubert Cancik and , Helmuth Schneider, English Edition by: Christine F. Salazar, Classical Tradition volumes edited by: Manfred Landfester, English Edition by: Francis G. Gentry. Consulted online on 09 September 2020 http://dx.doi.org/10.1163/1574-9347_bnp_e1402770 First published online: 2006 [paywall]