Toolkit is not using ResourceBundle to load properties(awt.properties.).

So it will not look for awt_<locale>.properties to load properties.
For example, it will not look for awt_ja.properties for ja locale.

Overwriting existing awt.properties doen't work for current l10n strategy.

Either adding message property files as an extension for each locale or
put all files in standard JDK release.

So, use ListResourceBundle and place the property file under something like
java.awt.resources package.