Sigismund is not a Latinized (or half-Latinized) form of the name.
Tacitus Latinizes SEGIMUNDUS.
It looks like there is an older form of the High
German word „Sieg“ (victory“): SIGIS, obviously Gothic and an inferred Germanic form, and there is a younger form:
SIGI, which is Old
Saxon or Old High
German SIGU (both from about 9th cent.). There seems to have been a bit of to an fro between the two in later centuries. A 5th cent.
Prince of
Burgundy was known both as
SIGISMUND and SIGIMUND. So the S was sometimes dropped a long time before. Confusion, wherever you look.