[Backport 7.9] - ERROR/stacktrace when adding compound field to document type.

Reproduction: Create, build and start new project from archetype. go to CMS Document Type Editor, create new document type, single row layout. Before saving/closing, open the Compound Field section and click on the imagelink (I don't think it matters which one you click). Terminal shows an ERROR message + stack trace, complaining about the absence of the hippotranslation:locale attribute on the document type's prototype node:

ERROR http-bio-8080-exec-3 [FieldPluginEditorPlugin$PropertyEditor.getClusterParameters:164] failed to add child node to plugin config
[INFO] [talledLocalContainer] javax.jcr.nodetype.ConstraintViolationException: /hippo:namespaces/ess379/test/hipposysedit:prototypes/hipposysedit:prototype: mandatory property

locale does not exist
[...]

[INFO] [talledLocalContainer] at org.hippoecm.repository.impl.SessionDecorator.save(SessionDecorator.java:179)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.editor.plugins.field.FieldPluginEditorPlugin$PropertyEditor.getClusterParameters(FieldPluginEditorPlugin.java:162)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.editor.plugins.field.FieldPluginEditorPlugin$PropertyEditor.onBeforeRender(FieldPluginEditorPlugin.java:137)

[...]