Performance: do not process untranslatable entities as translatable

If the entity is not translatable isn't \Drupal\Core\Entity\ContentEntityBase::getTranslationLanguages() only ever going to return the default language - right?\

The reason I ask is that I think we should do both the encrypt and decrypt if we're going to do this. Because when those are not mirrors it hurts my head :)

What sort of speed up did you get with this?

Note there is a performance script in the module that you use to prove this change is worth it.

Needs work and performance testing as per #4.