Can't open terminal in Ubuntu 18.04 after upgrade from 17.10

This mostly happens if the language setting on your system got messed up, to fix this simply re-set your language settings (see Screenshot).

After this your terminal should work as expected.

So I came across the same issue and no language settings seemed to help. Turnout the issue was with my PPA. I had recently upgraded my python3 to 3.7.5 from 3.6. This would have been fine but then I make my default python3 to be 3.7 which made some issues. So the solution to this is: Run this command

 sudo gedit /usr/bin/gnome-terminal

and than change #! /usr/bin/python3 to #! /usr/bin/python3.6 This seems to solve my isuue. Thanks!

This fix seemed to work for me. But did not survive a reboot.

When running localectl some entries showed as "n/a" - this seems to be key.

Any command-line activity can be run in xterm (Alt-F2, xterm).

Actual fix (for me):

1. Make sure /etc/locale.conf has your locale(s) uncommented.
2. Run locale-gen.
3. Run localectl set-locale LANG="en_US.UTF-8" (or you LANG value)
4. Run localectl to see if anything remains set to "n/a"
5. In my case, run localectl set-keymap "us"
6. Logout, login - now Terminal works.

(Credits to this thread: https://bbs.archlinux.org/viewtopic.php?id=180103)