String translation is now a service

As of Drupal 8, the string translation logic has been moved to a service and the translators that handle translations are now pluggable.
Core provides a few translators by default:

• CustomStrings Provides translation of custom strings set in settings.php
• FileTranslation Provides translation imported by po files
• StaticTranslation Provides simple translation by array of strings passed through its constructor

Locale module provides the full featured translation that it used to provide in Drupal 7 using locale's string storage and database caching.

Any module can provide additional translators by registering a service that implements the TranslatorInterface and is tagged as string_translator. A priority can be set if you want your translator to fire before other translators. (higher priority means your translator will fire first)

Example:
In mymodule.service.yml:

services:
  string_translator.mymodule:
    class: Drupal\mymodule\MyModuleTranslation
    tags:
      - { name: string_translator, priority: 5 }

All translators will fire sorted by priority until a translator that returns a translation is found.

Further improvements / API changes

- This also means that you can inject the translation service into your code, instead of relying on the global t() function, thus making your code unit-testable. (see https://drupal.org/node/2079611 exactly how)
The name of the new service is string_translation and can be accessed by procedural code using Drupal::translation()

- Locale's module storage has been moved to a service as well, named locale.storage and can be injected or accessed using the global Drupal object in procedural code:

D7

$storage = locale_storage();

D8

$storage = Drupal::service('locale.storage');