I just rebooted and for some reason am unable to switch keyboard layouts. This feature had been working previously since just after getting the computer (and setting up the alternate keyboard layout).
The secondary input source is shown here:
My attempt to select the alternate input source is shown here: note I clicked on usd2. However the input source did not change as a result.
I did not do anything related to keyboard settings for quite some time so it is puzzling this problem just showed up. I am on Sierra with MBPro 2015.
Best Answer
I re-added the same keyboard input:
.. and the second one is working.
I have no idea why this suddenly started failing.