Indicator – How to Install a Notification Indicator for Keyboard Layout

I'll tell you what eventually worked best for me.

1. Open the gnome preference "keyboard layout" pane
2. Move the layout order, if you have more than one
3. Delete all layouts except the default one (mine was a US layout)

The previous two steps have been suggested as fixes on their own (on the Ubuntu forums?). They did not work for me. I tried them together consecutively and this again did not work for me. Thus my next step.

1. Press, "Reset to Defaults"
2. Layout changes effectuated from both the layout pane and indicator now result in a change in layout.
3. Of course, add layouts back before attempting to change to them. Use the preview function to find a layout that easily produces the diacritical marks you seek.

I think my problem was from the fact that some of the layouts had changed in name, at least that's my take considering that the layout names weren't exactly what I expected when reassigning them. Deleting my extra layouts and then assigning new ones from a fresh list seems to have circumvented my issues, that being that the layouts of my choice now point to existing layouts with names to match.

As a workaround you may set an alternative keyboard shortcut like alt+shift to switch between keyboard layouts (see this question for reference: Ubuntu 17.10 can't change the input switching shortcut to alt+shift).

This shortcut should obey the "static ordering" you're looking for.

_{A feature request has been filed by OP here.}