Call to an undefined method Drupal\locale\StringStorageInterface::dbStringSelect().

Thanks for the report and potential fix! Our decorator does currently have to act in a way that means it can't be a pure decorator. Some of the problem with decorating has already been pointed out in #3203641: Don't consider locale fallbacks while editing translations → , with the suggestion that the service should be swapped out entirely rather than decorated. So to some extent, I'm impressed that Symfony Translation and Language Hierarchy are able to work together at all!

Given that we already use a parent:: call in our StringDatabaseStorageDecorator::dbStringLoad() implementation, there's also a clear precedent that your solution is probably the right way to go. I have done zero testing, and don't really have capacity to do any in the near future (just being honest!), but I feel positive about this change. I welcome reviews from others in order to progress this!