The Polish word ciwun is used in the English Wikipedia.
https://en.wikipedia.org/w/index.php?search=ciwun&title=Spec...

(tivun) ... = Junior provincial administrator
https://books.google.lv/books?id=bk5dDwAAQBAJ&pg=PR20&dq=tiv...
(in Frontiers in Question: Eurasian Borderlands, 700–1700, edited by Daniel Power, Naomi Standen)

Some other options which I would not prefer: “bailiff” and “castellan” seem incorrect or at least misleading/imprecise due to each having several meanings; and “tivun” points to the history of Russia instead of the Polish–Lithuanian Commonwealth.

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Note added at 5 hrs (2021-08-19 18:50:09 GMT)
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1. ‘valsčiaus valdytojas – kunigaikščio vietininkas’ = vicegerent. https://en.wikipedia.org/wiki/Vicegerent
2. ‘dvaro prievaizdas’ = castellan https://en.wikipedia.org/wiki/Castellan

Since it is not known precisely which of the two above is meant (and they are quite different but both existed in the Commonwealth), our last resort is to use the title “ciwun” which appears at least in some reference and academic publications — so the reader can eventually look it up if necessary.