The majority of Drupal 8's APIs are designed to support multilingual by default and make sane assumptions about common scenarios. As a result, there are several important things to keep in mind to build the best integration possible.
As you likely know, creating multilingual websites in Drupal 7—with its web of modules and dependencies—is not for the faint of heart. What takes Drupal 7 20+ contrib modules to support is now part of core in Drupal 8. Its 4 multilingual core pillars—Language, Interface, Content, Config—make Drupal 8 a relieving contrast to Drupal 7’s old patchwork of multilingual afterthought.