Example with CKEditor WYSIWYG inline images field.
Also see : Create a Form with Drupal 8 using Form API
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StringTranslationTrait allow to get t() and formatPlural() methods
Example:use Drupal\Core\StringTranslation\StringTranslationTrait;
use Drupal\Core\StringTranslation\TranslationInterface;
class MyClass {
Import configuration file programmatically on Drupal 9 using config_update contrib module and service
//Update Module settings
\Drupal::service('config_update.config_update')->revert('system.simple', 'mymodule.settings');