Oops, right. The string is required for Moodle 2.3 and therefore it is introduced in the Book master branch only. The stable 2.2, 2.1 and 2.0 branches do not have this string.
I just fixed AMOS database so that your translation appears at 2.3dev correctly. Thanks for pinging me.
Well, yes. It can be shown in your Moodle 2.2 if you installed Book's master branch instead of its MOODLE_22_STABLE branch. The point is that this capability is used by Moodle 2.3 and higher only. Therefore Petr correctly included the string in the master branch only and that one will form 2.3 release of the plugin.