Translation API - PHP Twig JavaScript

PHP Class using StringTranslationTrait

StringTranslationTrait allow to get t() and formatPlural() methods
Example:
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 fall back to \Drupal::translation()
    // but it is recommended to do so, for easier testability,
    $this->stringTranslation = $string_translation;
    $string = $this->t('Something');
  }
}

In annotation text

Examples:
@Translation("File")
@Translation("Text in context", context="Name of context")

In a Java Script (.js file)

Syntax: Drupal.t(str, args, options);
Examples:
Drupal.t('May');
Drupal.formatPlural('May');
Drupal.t('May', {}, {context: "Long month name"});

In Twig templates

Examples:
{{ 'Hello Earth.'|trans }}
{{ 'Hello Earth.'|t }}
{{ 'Hello @username.'|t({'@username': username}) }}
{% trans %}Hello {{username}}.{% endtrans %}
{% set labelText = '@action for @name'|t({'@action': actionText|default('Close')|t,'@name': nameText}) %}

Doc : https://www.drupal.org/docs/8/api/translation-api/overview