If a BBCode template uses language keys that don’t come from common.php (e.g. L_QUOTE which is in viewtopic.php), those keys will not be available, even if the dictionaries are added in the feed controller, because phpbb\textformatter\s9e\renderer::configure_user is called by the DI system before it’s possible for anything to add more dictionaries. (The default feed controllers also fail to add dictionaries that they need, but this is tracked in PHPBB3-16365.)

phpbb\feed\helper::generate_content already makes calls to reconfigure the other parts of the renderer; it simply needs to call configure_user too.