Dedicated method to get the entity langcode

Entity benefits from a new method, ::getLanguageCode(), that allows to retrieve the language code of the entity. Until now, the language code was obtained from the entity ::language() method.

Before (not recommended)

Until now we had to get first the language object from the entity and then obtain its ID (language code). This way of getting the language code is still working but it's not recommended.:

$langcode = $entity->language()->getId();

After (recommended)

$langcode = $entity->getLanguageCode();