Change record status: 
Project: 
Introduced in branch: 
8.x
Description: 

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

<?php
$storage
= locale_storage();
?>

D8

<?php
$storage
= Drupal::service('locale.storage');
?>
Impacts: 
Module developers
Updates Done (doc team, etc.)
Online documentation: 
Not done
Theming guide: 
Not done
Module developer documentation: 
Not done
Examples project: 
Not done
Coder Review: 
Not done
Coder Upgrade: 
Not done
Other: 
Other updates done