[BeckyEbook]

From reading the Qt windows platform code, it looks like a left Ctrl and right Alt should be detected as an AltGr if no altgr key exists. Of course, I have not tried that at all!

The real problem with AltGr is that it is treated as Ctrl+Alt.
Imagine you have the shortcut AltGr+4=€
I can also get it by pressing Ctrl+left Alt+4.
But ... in Sigil, adding an environment variable or parameter -platform windows:altgr makes us sort of separate these two things from each other and thus:
AltGr works normally and is used to obtain additional characters (usually diacritics), and all shortcuts from Ctrl+left Alt can be used for keyboard shortcuts.

I didn't add the code, I just added a checkbox to the src\Form_Files\PKeyboardShortcutsWidget.ui file (as demo mockup).

Changed fragment:

Spoiler:

   <item>
    <layout class="QHBoxLayout">
     <item>
      <widget class="QPushButton" name="resetAllButton">
       <property name="toolTip">
        <string>Reset all to default</string>
       </property>
       <property name="text">
        <string>Reset All</string>
       </property>
      </widget>
     </item>
     <item>
      <spacer>
       <property name="orientation">
        <enum>Qt::Horizontal</enum>
       </property>
       <property name="sizeHint" stdset="0">
        <size>
         <width>40</width>
         <height>20</height>
        </size>
       </property>
      </spacer>
     </item>
     <item>
      <widget class="QCheckBox" name="EnableAltGr">
       <property name="text">
        <string>Enable AltGr on international keyboard</string>
       </property>
      </widget>
     </item>
    </layout>
   </item>

EDIT: I already have a working code.