Provide language entity property for all translatable entities

When working with Rules and Views VBO I couldn't find a way to set the language for taxonomy_terms.
Because entity_translation relies on this property for showing the 'translation tab' I thought this would be the place to expose this property.

I do have some questions about this approach:
- It relies on Entity API, which is not a dependency of this module.
- I am not sure how well this works for entities without a language property in their schema. For instance i18n_taxonomy adds this field to the taxonomy schema.