There are two examples of plural forms on this page
"%d key" : [ "%d key", "%d keys" ],
"test singular": ["test plural", "test_1 singular", "test_1 plural"],
These appear to be different structures. Using Gettext terminology they look like:
msgid: [ msgstr1, msgstr2 ]
msgid: [ msgid_plural, msgstr1, msgstr2 ]
Can you please clarify how this structure should be used and whether it has changed at some point?