I just installed the newest LTS (18.04) and ran into this problem while setting it up. I need to be able to type in Korean (Hangul); I have been using Korean in 16.04 on two machines for the past few years, and it has worked without a problem.
After Korean is added as an additional language input sources, I cannot use it. I am able to select/toggle the various input languages via the dashboard icon and using super+shift; however only English characters appear when hitting keys.
For troubleshooting, I also added Hebrew, Hungarian, and Icelandic. The Hebrew works perfectly when selected. Both Hungarian and Icelandic only have a few characters changed which may be the default functionality of those layouts; I'm not familiar with them so I cannot be sure. As per the standard, right-alt should toggle between English and Korean characters, but it does not.
Additionally, I booted into my Windows partition and the right-alt key worked fine, so it is almost certainly not an issue with the actual keyboard.
Lastly, I have also tried using "gnome tweaks" -> "Keyboard & Mouse" -> "Additional Layout Options" -> "Korean Hangul/Hanja keys" to explicitly set the right-alt functionality, but it still doesn't work.
Any aid would be greatly appreciated! If I can't get Korean working, I'll be forced to go back to 16.04 (or Windows) and I really would prefer not to.