ContentEntityInterface::getTranslation() should throw an exception when an invalid language is specified

Problem/Motivation

ContentEntityInterface::getTranslation() returns a new translation if the specified language is valid but there's no corresponding translation for it yet. This is not ideal, we should be creating new translations explicitly. Moreover this has also performance implications, since ContentEntityInterface::addTranslation() is slow (see also #2382675: hook_entity_create() affects the data of new translations of existing entities in unexpected and undocumented ways → ).

Proposed resolution

Remove the ContentEntityInterface::addTranslation() invocation from ContentEntityInterface::getTranslation().

Remaining tasks

• Figure out what's the desired behavior when LanguageInterface::LANGUAGE_NOT_SPECIFIED is passed.
• Reviews

User interface changes

None

API changes

Yes

Data model changes

None

Beta phase evaluation