The warning log about missing labels ( ZK-2075) in https://github.com/zkoss/zk/blob/v8.5.2.1/zcommon/src/org/zkoss/util/resource/impl/LabelLoaderImpl.java#L95 can be quite verbose at times and even have a considerable performance impact.

In some use cases it might be intended to allow missing labels, and implement the missing label fallback inside the application code (e.g. if labels are not only locale dependent).

As the intention of this log is for debugging purposes the log level can be changed to DEBUG, to avoid affecting production system logs, which often run at INFO or WARN level.

At the same time a protected method to handle the missing label can be added (e.g. "handleMissingLabel") method to allow easy overriding and configuration of a custom LabelLoaderImpl implementation (The default constructor can't be bypassed by subclasses)