Drupal 7 and earlier supported loading interface translation (.po) files from /translations subdirectories under project directories. This resulted in conflicts with version control, where translation files were looked for at places, where they were not shipped in packages or via git. To avoid confusion, simplify this system and make translation deployments easier, Drupal 8 centralizes translations directories in one.
There is also a stream wrapper introduced for this directory, so it is possible to have differently configured directories in staging and development and still refer to files with the same path. You can access files in this directory with the 'translations://' stream, much like the public and private file stream wrappers:
$po_file_path = 'translations://drupal-8.0.hu.po';