Supporting infrastructure for standalone localization plugins. This plugin doesn't provide notable end user features, and will be installed as dependency of other plugins that use the features provided by this plugin.
Jenkins supports multiple ways to provide localized resources, and each of them has a corresponding code path in this plugin.
The localizer library looks up Messages_??.properties files in packages for the generated Messages classes. Support for localizing messages from the localizer library is added via the io.jenkins.plugins.localization.support.localizer package in this plugin.
Stapler views look up viewname_??.properties files in directories with the same name as the view. io.jenkins.plugins.localization.support.stapler.ResourceBundleFactoryImpl is a ResourceBundleFactory that gets installed by StaplerManager. I ends up calling LocalizationContributor#findResource(String, Class) with a cleaned up String argument.
Descriptor#doHelp will serve corresponding help.jelly views, if they exist, and lets Stapler localize them (see above).
Descriptor#doHelp looks up com/acme/package/MyDescribable/help-fieldname_??.html HTML files and serves them at /descriptor/myDescriptorSymbol/help/fieldname, typically corresponding to specific fields in views. It also supports com/acme/package/MyDescribable/help_??.html at /descriptor/myDescriptorSymbol/help.
Localization support of these is accomplished through setting MetaClassLoader#debugLoader to a classloader that has localization plugins on its class path.
Jenkins core exposes war/src/main/webapp/ resource files directly packaged into the war file via Stapler#service invoking #openResourcePathByLocale. This in turn ends up invoking LocaleDrivenResourceProvider#lookupResource, an SPI implemented in Jenkins core as MetaLocaleDrivenResourceProvider from 2.173. This in turn looks up implementations of PluginLocaleDrivenResourceProvider in plugins. There is one, PluginLocaleDrivenResourceProviderImpl in localization-support.
Plugins expose src/main/webapp/ resource files directly packaged into the jpi file via Plugin#doDynamic at /plugin/namehere/ invoking StaplerResponse#serveLocalizedFile. This calls #selectResourceByLocale which ends up invoking LocaleDrivenResourceProvider#lookupResource. See the previous section for further details.
The only user feature exposed by this plugin is an administrative monitor that complains when an incompatibility is detected. Some of the mechanisms used to provide localized resources from implementing plugins are exclusive, and other plugins interfering with that would prevent this plugin from working. If such a case is discovered, the administrative monitor informs users about the problem.
Implementing plugins need to provide the localized resources in the same general directory layout as they would be placed in core and individual plugins.