Seems to be a bug. I'll explain how to get out of it... as long as you have both US and INTL (US International) input methods available, but first...
Here's how I found it... (CAUTION: DON'T DO THIS IF YOU ARE NOT WILLING TO BE ANNOYED) First, go to US keyboard entry method, while having INTL also available. Now, hold the right Alt key, and press the shift key (tested with left-shift). Now, you are in US-International mode, and the right Alt key you are holding down, is no longer an alt key.... so when you release it, the system gets an AltGR released signal, rather than an Alt key being released. Now if you press and release Shift, it will cycle input methods.
Okay, now... to get out of that annoying mode, select INTL input method. Hold Alt-GR (Right Alt) and press Space as many times as it takes to select US input method. Then release the Right-Alt key (Alt-Gr). Note: This MAY HAVE OTHER SIDE EFFECTS, since the system didn't get any signal that Alt-Gr was released. I haven't noted any such side-effects, but that doesn't mean there aren't any.
To anyone who might try to fix this bug: While you're at it, please make Ctrl-Space do something more useful, such as cycling input methods in reverse, or switching to the last input method that some key was pressed in other than Shift, Alt, and Ctrl. As it is right now, with more than 3 keyboard layouts, there is NO QUICK WAY to switch back and forth between an arbitrarily chosen pair of available input methods. I'm using multiple input methods for DuoLingo, and it's a pain to have to press Alt-Shift Alt-Shift Alt-Shift Alt-Shift Alt-Shift to switch languages, and then Alt-Shift Alt-Shift Alt-Shift Alt-Shift Alt-Shift Alt-Shift Alt-Shift Alt-Shift to switch back, as I am asked to translate to and from a course's target language.
(If anyone doubts it's that bad, look at how many language courses I'm taking. Username DonaldKron --- and what it shows as the courses I'm taking, is actually not all of them.)