Drupal::languageManager()->getLanguage(Language::TYPE_INTERFACE) is a bit ugly to use.
Let's add helper methods for the hardcoded types (the Language::TYPE_* constants), e.g. getInterfaceLanguage(). Method should be very simple, just call getLanguage($type) internally.
This saves a use of the Language class where we just need it for that and is much easier to understand with method autocompletion.