Created attachment 9326 
Patch

Steps to reproduce:
0. Use a system that does not use dot (.) as decimal separator.
1. Open the screensaver preferences (`xfce4-screensaver-preference`).
2. Select any screensaver within the "Theme" listbox.
3. Click the "Configure screensaver" button which will open the configuration dialog for the particular screensaver.
4. Change any numerical parameter (e.g. the frame rate).
5. Close the configuration dialog.
6. On the main screensaver preferences dialog, click the "Configure screensaver" button once again.

Results:
The configuration dialog (`xfce4-screensaver-configure`) is supposed to be shown just like the first attempt. However, it does not. Instead, it terminates when it attempts to parse a double-type parameter value given by the `xfconf-query` program, which happens to respects the system's locale preferences:

[stdout of xfce4-screensaver-configure]
Traceback (most recent call last):
  File "/usr/bin/xfce4-screensaver-configure", line 756, in <module>
    configure(obj.to_dict())
  File "/usr/bin/xfce4-screensaver-configure", line 678, in configure
    win = ConfigurationWindow(parsed)
  File "/usr/bin/xfce4-screensaver-configure", line 395, in __init__
    widget = self.get_option_widget(opt)
  File "/usr/bin/xfce4-screensaver-configure", line 488, in get_option_widget
    value = self.get_double(opt["id"], opt["default"])
  File "/usr/bin/xfce4-screensaver-configure", line 518, in get_double
    value = self.xfconf_channel.get_double(option_id, default)
  File "/usr/bin/xfce4-screensaver-configure", line 83, in get_double
    return float(self._get_property(prop, default))
ValueError: could not convert string to float: '8000,000000'
[end]

Possible solution:
Instead of using `float` function to parse the given double-type value, use the `atof` function from Python built-in `locale` module, which respects the system's locale preferences (see attached patch).