Handle i18n_strings

This could do with some tests.

Adding a tag so someone at ComputerMinds can pick this up.

@james.williams thanks very much for the patch and tests!

Could you provide a patch that has just the tests please, and not the fix/feature then we can see that the tests do indeed catch the fact that we don't get the expected behavior.

Also, a quick review of the patch:

+++ b/tests/language_hierarchy.test
@@ -378,3 +378,121 @@ class LanguageHierarchyStringTranslationWebTestCase extends LanguageHierarchyBas
+    // Check if i18n_string() works as expected.
+    $parent_translation = i18n_string_translate($name, 'NOT FOUND', array('langcode' => $parent_langcode));
+    $this->assertTrue($string != $translation && $parent_translation == $translation, 'i18n_string() works for parent.');
+
+    // Check if inheritance of i18n_string() works as expected.
+    $child_translation = i18n_string_translate($name, 'NOT FOUND', array('langcode' => $child_langcode));
+    $this->assertTrue($string != $translation && $child_translation == $translation, 'i18n_string() inherited from parent.');
+
+    // Check if translated string for child language works as expected.
+    $child_translation2 = i18n_string_translate($name, 'NOT FOUND', array('langcode' => $child_langcode2));
+    $this->assertTrue($string != $translation2 && $child_translation2 == $translation2, 'i18n_string() with translation for child.');

I'd suggest changing the comments here to indicate what is expected, rather than leaving the reader to work out what the intention was. Something like:

// Check if inheritance of i18n_string() works as expected: no specific translation for this child language, so translation in parent language returned.

etc.

Lovely stuff, thanks for the patch.