Drupal 8 loads alpha2 translations instead of alpha12

Problem/Motivation

Currently Drupal 8-dev is not packaged with version numbers and git downloads don't have version numbers obviously. Neither a proper git_deploy module to use to install. The installer has a hardcoded fallback for translations to alpha2 though, so when you install Drupal 8, translations for the second alpha are downloaded. Unfortunately there has been lots of changes since the second alpha and a thousand new string have been identified in recent updates in alpha12, see https://localize.drupal.org/node/6433.

Proposed resolution

Ideally we would have a server side fallback mechanism and no need to update manually hardcoded things. Revisit before beta was removed from #2113955: Rely on proper server side version fallback for translations though and its definitely not prioritised anywhere near critical to work on. That means we are going with manually hardcoded fallbacks I guess unless someone has time for that issue. So let's update the manually hardcoded number then.

Remaining tasks

Review. Commit.

User interface changes

Finally more up to date translations will be downloaded.

API changes

None.