Abtract translation set concept to work with entities in general

Drupal core includes translation set support in 6 and 7. This is good for cases where you need separate permissions, workflows, authors, import/export, etc. for language versions. Also if you need different relations, like put them into different menus. (Field translation is great when you want to use the same workflows, permissions, relations, etc). Translators usually choose between the two models per content. For example, OG entities (nodes) would use field translation because they have users related to them for signup, commerce entities would use field translation because then you don't need product copies for each language. If you have press releases translated via outside companies with a common EU workflow for example to require separate editing permissions and workflow to publish them at once when certain language version are available as required, you'll use different entities. Also, if you want to use different language menu trees, manage revisions separately etc.

These two systems should live on separately for the different use cases, but the node translation set system is very simple and should be extended to other entity types. This might or might not be solvable with a more generic relationship solution.