Drupal 8 does not intend to support two parallel content translation systems. The legacy node translation module stores nodes as separate entities when translated. The new content translation module stores translations all in the same node. Consider adding an upgrade path to migrate this data to the new system.
Beware: other related data such menu items, taxonomy term relations, node access rules, path aliases, visitor stats, etc. will be affected, and would need to be taken care of. This is likely what makes the task *huge*!
Bring the existing migration code up to date from http://drupal.org/project/entity_translation and make it work in Drupal 8. See if we can tackle the related data in core and provide contrib with ways to cope with the extent of the change.
Nope. Won't fix by @Dries.
I'd be ok with fixing the upgrade path [of content translation] in contrib.
BUT, also at Prague it was suggested to not have upgrade path in 8.0 .. in general, not just for this old content translation. (Need a post, discussion or issue to link to that explains that plan.)
(optional, will need to migrate node access data as well accordingly)