Pick up library language per item, with optional global override.
Replaces the fixed PreferredLanguage config field with PreferredLanguageOverride (default empty). When empty, each item resolves its preferred language via BaseItem.GetPreferredMetadataLanguage(), the same resolution chain Jellyfin uses everywhere (item override, then parent hierarchy, then library settings, then server default). This is what issue #8925 actually asks for: respect the library language, not a hardcoded one.
The override is still there for users who want to force a single language across all libraries.