Is this not better?

JLoader::register('ModulesDispatcher', JPATH_ADMINISTRATOR . '/components/com_modules/dispatcher.php');

$app             = \Joomla\CMS\Application\CmsApplication::getInstance('administrator');
$app->loadLanguage($this->app->getLanguage());
$dispatcher      = new ModulesDispatcher('Joomla\\Component\\Modules', $app, $input);
$controllerClass = $dispatcher->getController('Module');

// Execute backend controller
$return = $controllerClass->save();

The problem is that when we create dispatcher object;

$dispatcher      = new ModulesDispatcher('Joomla\\Component\\Modules', $app, $input);

The loadLanguage() method is called immediately https://github.com/joomla/joomla-cms/blob/4.0-dev/libraries/src/CMS/Dispatcher/Dispatcher.php#L93

The $app property of dispatcher object now is Administrator application object, it doesn't have $language property available yet, so $this->app->getLanguage() return null, thus it causes fatal error as I described

Ah, sorry. Maybe your code works But in this case, language will be loaded twice I think. Will need to try it

OK. @wilsonge code works (although not easy to figure out). Let me know if you want to have the code like that and I will close this PR.

@mbabker thoughts please

My gut is that setting the existing language into the application is superior. But willing to listen to other arguments

BTW, loadLanguage() method in Dispatcher is not implemented properly. We are hardcode the path to JPATH_BASE and JPATH_COMPONENT which is wrong in the given sample code (the path must he JPATH_ADMINISTRATOR and JPATH_ADMINISTRATOR . '/components/com_modules';)

Agreed. I think JPATH_BASE is correct. But JPATH_COMPONENT is wrong

JPATH_BASE is not correct, too. With that sample code, the active component is com_config in the frontend, so JPATH_BASE equals JPATH_ROOT. But when we call com_modules backend, we need to language file from JPATH_ADMINISTRATOR, so JPATH_BASE is wrong

Unless application class has a method such as getPath() to return the root path of the application, I think we will need to use if else command here

Ugh