Programmatically create a menu link in Drupal 8

create_menu_link.php

<?php

use Drupal\menu_link_content\Entity\MenuLinkContent;

MenuLinkContent::create([
  'title' => 'Hello World',
  'link' => ['uri' => 'internal:/node'],
  'menu_name' => 'main',
  'weight' => 10,
])->save();

How can i create a menu link that is child of other menu link ?

You need to provide a parent plugin ID.
e.g.

use Drupal\menu_link_content\Entity\MenuLinkContent;
// $parent_link = Drupal\menu_link_content\Plugin\Menu\MenuLinkContent
MenuLinkContent::create([
  'title' => 'Hello World',
  'link' => ['uri' => 'internal:/node'],
  'menu_name' => 'main',
  'weight' => 10,
  'parent' => $parent_link->getPluginId(),
])->save();

To get the parent MenuLinkContent link item, look at the plugin.manager.menu.link or menu.link_tree services

Where that code should be placed so it's executed only once? Also, what about translatable menu items? How to add menu item that has different title and url (path) for different languages?