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+# coding=utf-8
+
+"""Add internationalisation support to a Sugar activity.
+
+Below is a simple Sugar activity which displays a different string every time
+the button is pressed. You must modify it to add internationalisation support
+so that its UI can be viewed in a different language.
+
+All the other files for the activity (activity.info, setup.py, etc.) have been
+provided already; you must modify the code in this file to add
+internationalisation support, generate a POT file, and then translate it to
+Spanish.
+
+Note that the original strings in the program are always in English, by
+convention.
+
+ 1. Add an ‘import’ statement for the gettext module:
+        from gettext import gettext as _
+ 2. Mark all the relevant strings in the program as translatable using the _()
+    function.
+ 3. Generate a POT file by executing ‘./setup.py genpot’ in a terminal (after
+    changing to this directory).
+ 4. Copy the generated po/Exercise19.pot file to po/es.po.
+ 5. Either open es.po in a text editor, or use a translation tool like
+    gtranslator, and translate the strings to Spanish.
+ 6. Run ‘./setup.py dev’ to add the activity to Sugar on your computer.
+ 7. Run ‘./setup.py build’ to compile the .po files into machine-readable .mo
+    files in the ‘locale’ directory.
+ 7. Run ‘LANG=es sugar-emulator’ to run the Sugar emulator in Spanish and test
+    the new translation. The whole activity should appear in Spanish.
+
+Note: It is not possible to use Pootle for this exercise because it’s an online
+service and would require the exercise’s code to be uploaded to the internet.
+"""
+
+from gi.repository import Gtk
+from sugar3.activity import activity, widgets
+from sugar3.graphics.toolbarbox import ToolbarBox
+from gettext import gettext as _
+
+
+class Exercise19Activity(activity.Activity):
+    """A simple window which displays a label and a button."""
+    def __init__(self, handle):
+        super(Exercise19Activity, self).__init__(handle)
+
+        # Create the standard activity toolbox.
+        toolbar_box = ToolbarBox()
+        self.set_toolbar_box(toolbar_box)
+        toolbar_box.show()
+
+        main_toolbar = toolbar_box.toolbar
+
+        activity_toolbar_button = widgets.ActivityToolbarButton(self)
+        main_toolbar.insert(activity_toolbar_button, 0)
+        activity_toolbar_button.show()
+
+        stop_button = widgets.StopButton(self)
+        stop_button.show()
+        main_toolbar.insert(stop_button, -1)
+
+        # Add a box to the window.
+        box = Gtk.Box()
+        box.set_orientation(Gtk.Orientation.VERTICAL)
+        box.set_spacing(8)
+        box.set_border_width(8)
+        box.show()
+        self.set_canvas(box)
+
+        # Set up a list of text strings to display in the label.
+        self._strings = (
+            _('Initial string.'),
+            _('Second string.'),
+            _('Handle object is ‘%s’.') % handle,
+            _('Another string.'),
+        )
+        self._next_string = 0
+
+        # Create and set up the label and button.
+        label = Gtk.Label()
+        label.show()
+        box.pack_start(label, True, True, 0)
+
+        button = Gtk.Button(_('Change Label'))
+        button.show()
+        box.pack_start(button, False, True, 0)
+
+        # Connect the button callback.
+        button.connect('clicked', self.__button_clicked_cb)
+
+        # Store the label as an object variable so it can be accessed from the
+        # button callback, and then set the initial text for the label.
+        self._label = label
+        self.__rotate_label_text()
+
+    def __button_clicked_cb(self, button):
+        """Callback to change the label text when the button is clicked."""
+        self.__rotate_label_text()
+
+    def __rotate_label_text(self):
+        """Change the label to display the next string in the rotation.
+
+        The modulus operation is used to wrap self._next_string around when
+        it reaches the final index in the self._strings array.
+        """
+        self._label.set_text(self._strings[self._next_string])
+        self._next_string = (self._next_string + 1) % len(self._strings)