Support for multi-language documents

Currently the workflow only supports single-language documents.

If the workflow is used in a multi-language wiki, one can edit and push documents through the workflow process, but only the default language will be published.
Also the PublicationWorkflow.Overview page shows all language variants, and on publishing some documents first displays an error, and then on the second attempt (in a two-language wiki) when only one variant is displayed, allows to publish the default version.

It would be better of all language variants are taking into account.
As language variants cannot have objects of their own (and thus not separate workflow states) the technical simplest solution would be to review/validate/publish all language variants at once.
Maybe a note to the publication panel - if the current document has several language variants - that all languages will be published will minimize the element for surprise for these users that assume a per-language workflow.