Make FlexSlider rendered entity language aware

Problem/Motivation

In a multilingual site referencing in a node a flexslider rendered entity wich is field translatable, rigth now, the referenced entity is always rendered in its original language even if it have translations available.
Ideally, the referenced entity should be in the same language than the current node referencing it.
This is the same problem explained in #1674792: Rendered entity is not language aware for 'Entity Reference'.

Proposed resolution

Implement the same solution done for Entity Reference module.
Add an " Use current content language" option for the field formatter.