This is a regression introduced in 7u13:
A new localizedLevelName field was added to j.u.l.Level, which changed the serial form.
Once set, Level.getLocalizedName() will always return the cached name, regardless of the current locale.
The change in serial form should not introduce any interop issue, because localizedLevelName will be recomputed if null.
The fact that once cached, the localizedLevelName is never recomputed is the more problematic issue.