Up to date as of October 16th, 2015.
Once you install Drupal 8 in a foreign language, you'll have Language and Interface translation modules enabled with the chosen language configured. Drupal 8 has more core modules handling language related features, yet less requirement for contributed modules to be installed for the most important tasks (on my last count, the 4 modules explained here cover functionality of 20+ modules from Drupal 7 and in much better ways).
Why have multiple modules when a multilingual site just needs all the features? Well, there are also foreign language (not multilingual) sites that we aim to support better and multilingual sites can be very different as well. Also, admittedly there are technological reasons to organize the modules by the features they provide.
In Drupal 8 multilingual is one of the groups of core modules, so you'll find these modules under the main core modules in a neat group.
The Language module serves as the base layer for language support. The role of this module is to provide language configuration, assignment and detection functionality. We'll go into details about the changes around these areas in later tidbits. Suffice to say for now that if you have a multilingual blog with an English only interface but posts in different languages, enabling the Language module will be the only thing you need. It will let you assign languages to content, which is what you need.
Once you need your website interface to speak different languages, you'll need the Interface Translation module. (The combination of Language and Interface translation modules is what was Locale module in Drupal 7, but the two new modules have a lot more useful features and expanded on their reach a great deal in providing you support for your work.)
The Configuration Translation module has the sole purpose to provide a user interface to the existing backend support for configuration translation that is native to Drupal 8. Configuration is treated very differently in Drupal 8 compared to Drupal 7. A unified system exists to ship and update default configuration (similar but in several ways improved to how Features module worked in Drupal 7), so we treat configuration as part of the shipped code as far as language support is concerned.
A single foreign language website for example will typically have the Language and Interface Translation modules enabled with a single language configured (unlike a multilingual blog, where typically multiple languages will be configured but possibly no interface translation or configuration translation needed).
Finally, Drupal core also already ships with a Content Translation module. Although the name is the same, this is far from being the same as the Drupal 7 core module of the same name. The new content translation module works entirely differently, supports all content entities and translates fields. It conveniently makes numerous contributed modules obsolete!
This was just a short intro to the core modules. We'll go into details in later posts.
Issues to work on
- The reason for so few module groups is that we can only group modules that are single purpose. There is a generic tagging system proposed for modules to be able to be in different "categories" at once, but not a lot of activity on that recently. See https://drupal.org/node/1868444
The issue to track to add the configuration translation feature is http://drupal.org/node/1952394. This would be important for single language foreign language sites for highly increased usability and vital for multilingual sites. Drupal core's issue queue thresholds rules currently make it impossible to get this committed. See http://comm-press.de/en/blog/thresholds-code-freeze-getting-features-dru.... Previously the excessive delays in the base configuration language support landing blocked this issue.
The removal of the legacy content translation module would let us clean up so many things, including the node schema, however there are many dependencies (including an upgrade path): https://drupal.org/node/1952062