commit 5c44ae7da8dcf44a52c8984a78db2a3f72453d7e
Author: Erik Stielstra <info@erikstielstra.nl>
Date:   Mon Feb 11 14:20:50 2013 +0100

    WIP

diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index ec5ec0e..aa38a8d 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -1522,37 +1522,85 @@ function install_check_localization_server($uri) {
 /**
  * Gets the core release version for localization.
  *
+ * @todo Modify description.
  * In case core has a development version we fall back to the latest stable
  * release. e.g. 8.2-dev falls back to 8.1. 8.0-dev falls back to 7.0. Fallback
  * is required because the localization server only provides translation files
  * for stable releases.
  *
  * @return array
- *   Associative array containing 'core' and 'version' of the release.
+ *  Array of releases each is an associative array containing:
+ *   "core": Core compatibility version. e.g.. 8.x
+ *   "version": Release version. e.g. 8.1
  */
 function install_get_localization_release() {
-  if (strpos(VERSION, 'dev')) {
-    list($version, ) = explode('-', VERSION);
-    list($major, $minor) = explode('.', $version);
-
-    // Calculate the major and minor release numbers to fall back to.
-    // E.g. 8.0-dev falls back to 7.0 and 8.2-dev falls back to 8.1.
-    if ($minor == 0) {
-      $major--;
-    }
-    else {
-      $minor--;
+  $extra_text = $extra_number = '';
+  $releases = array();
+
+  // Examples of supported versions:
+  //   8.0-unstable1, 8.0-alpha2, 8.0-beta3, 8.0-rc4
+  //   8.0, 8.0-dev
+  list($version, $extra) = explode('-', VERSION);
+  list($major, $minor) = explode('.', $version);
+
+  // Extra may contain an index number. E.g. dev, alpha2, rc1
+  if ($extra) {
+    $matches = array();
+    preg_match('/([a-z])+([0-9]*)/', $extra, $matches);
+    $extra_text = $matches[1];
+    $extra_number = $matches[2];
+  }
+
+  // Stable point releases (no 'rc', 'beta', etc.):
+  //   8.2 -> 8.2 (no fallback, use current release)
+  //   8.2 -> 8.1
+  if (empty($extra)) {
+    $releases[] = array(
+      'core' => "$major.x",
+      'version' => "$major.$minor",
+    );
+    if ($minor >= 1) {
+      $releases[] = array(
+        'core' => "$major.x",
+        'version' => "$major." . $minor - 1,
+      );
     }
-    $release = "$major.$minor";
   }
   else {
-    $release = VERSION;
+    // Dev releases:
+    //   8.2-dev -> 8.1
+    //   8.0-dev -> none (falls back to 7.0, see final fallback below).
+    if ($extra_text == 'dev') {
+      if ($minor >= 1) {
+        $releases[] = array(
+          'core' => "$major.x",
+          'version' => "$major." . $minor - 1,
+        );
+      }
+    }
+    // Release candidate releases:
+    //   8.0-rc2 -> 8.0-rc1
+    //   8.0-rc1 -> none (falls back to 7.0, see final fallback below).
+    elseif ($extra_text == 'rc') {
+      if ($extra_number >= 2) {
+        $releases[] = array(
+          'core' => "$major.x",
+          'version' => "$major.$minor-rc" . $extra_number - 1,
+        );
+      }
+    }
+    // Other releases (unstable, alpha, beta) only fall back to the final
+    // fallback.
   }
 
-  return array(
-    'core' => "$major.x",
-    'version' => $release,
+  // All releases have a final fallback in the first previous major release.
+  // E.g. 8.1 falls back to 7.0
+  $releases[] = array(
+    'core' => $major - 1 . '.x',
+    'version' => $major - 1 . '.0',
   );
+
+  return $releases;
 }
 
 /**