This has been brought up before: ALT modifier bug in Windows
The problem is [juce_win32_Windowing.cpp line 976] (https://github.com/julianstorer/JUCE/blob/master/modules/juce_gui_basics/native/juce_win32_Windowing.cpp#L976), if you comment that line out then right-Alt will work. But note that some keyboard layouts map that key to AltGr instead, which will be reported to applications as Ctrl + Alt.
The opposite is also true, if your keyboard layout allows you to type accented letters with AltGr then you can type the same letters with Ctrl+Alt, for instance on my layout both AltGr + A and Ctrl + Alt + A will produce an accented letter “á”.