Activating the option "Allow selection of multiple directories" does not allow one to select more than one directory when running on Linux with Gnome 3. In this case the native file dialog is shown, but still allows only the selection of a single directory.

The code at https://github.com/metabrainz/picard/blob/master/picard/ui/util.py#L88 assumes on Linux multiple directory selection will always be possible, but at least when the Gtk3 dialog is used this seems not to be the case.