fun createJosmPluginJarManifest(): Map<String, String>
Returns a map containing all manifest attributes, which are set. This map can then be fed into org.gradle.api.java.archives.Manifest.attributes. That's already done automatically by the gradle-josm-plugin, so you normally don't need to call this yourself.
Make sure the langCompileTask already ran before this method is called, otherwise not all translated descriptions are included in the Manifest.