The current signing API does not support submission of the first listed add-on version, due to required extra meta-data. To remove the obstacles and avoid breakage in the release process, we could add an API that support this specifically for langpack submissions from the releng account.

When a the first listed version gets submitted through the API, AMO will set the required metadata to some hard-coded defaults, for example setting the license etc.

/cc @escapewindow