Add language plugin

[agentrickard]

At DrupalCON, someone requested a Language plugin for access control. That seems to make sense as a default option.

We'd have to look at how Content Translation stores data on nodes, but it should be possible.

[agentrickard]

This is interesting, because of how translations are handled in D8, which is at the field level. That means we can restrict access based on the language of the parent node, but not at the field translation level.

That means that access rules would apply to whatever language the node was first created in.

[agentrickard]

Not required for stable release.