Language entity missing uri() method implementation

Problem

The language entity is missing a uri() method implementation and therefore poses as having a generic /entity/* URL for the entity, which is not at all the case. Other config entities provide the uri() for the backend editing form for the entity, such as Block, View, etc. Config translation module uses this to add the additional Translate operation to listings for the entities. The lack of uri() implementation on Language entities makes the Translate operation go to the generic /entity/* URL which will obviously not work.

Proposal

Implement the uri() method exactly like Block.

Marking as a blocker for #2044389: Fix broken configuration translation pages which is a blocker for #1952394: Add configuration translation user interface module in core.