I needed to be able to type diacritics with my keyboard on Éowyn. I installed the "English (intl., with AltGr dead keys)" keyboard, but when I typed in the correct key bindings, such as AltGr + a, the character "á" was not produced.
The issue was that the Compose key was not set. Hence,
I had to set the Compose key to the
AltGr key.
I did this by installing the gnome-tweaks utility with
sudo apt install gnome-tweak-tool and ran it with
gnome-tweaks. In Tweaks,
I went to Keyboard & Mouse →
Compose Key. The default is set to off,
but you can toggle that switch and pick the key you want. I went with
AltGr, since that was what the keyboard I downloaded said to use.
You can do a lot of other cool things with Tweaks - you can customize how pretty much everything looks. You can change the fonts the OS uses, what format the time should be printed in, pick which apps should be launched at boot up, and so much more (and I realized I sound like an ad when I say this).
Future note: this works quite well on GNOME, but it doesn't work so well on GNOME-based desktop environments like Cinnamon. Check out my entry on mapping keys for more info.