How to support translation in your custom service with drupal 8-9

In your custom_module/src/myCustomService.php class file, modify your custom service and add Dependency Injection of Drupal\Core\StringTranslation\TranslationInterface like the example below:

use Drupal\Core\StringTranslation\StringTranslationTrait;
use Drupal\Core\StringTranslation\TranslationInterface;
class MyClass {
  use StringTranslationTrait;

  /**
   * Constructs a MyClass object.
   *
   * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
   *   The string translation service.
   */
  public function __construct(TranslationInterface $string_translation) {
    // You can skip injecting this service, the trait will fallback to \Drupal::translation()
    // but it is recommended to do so, for easier testability,
    $this->stringTranslation = $string_translation;
  }

  /**
   * Does something.
   */
  public function doSth() {
    // ...
    $string = $this->t('Something');
    // ...
  }

}

You custom.services.yml should look like this

services:
  api_charts.data:
    class: Drupal\custom_module\CustomDataService
    arguments:
      - '@string_translation'

    arguments:
      - '@string_translation' is a mandatory argument.