The use of dead keys to type accents does not seem to work in any progam. I have physical keyboard MICROSOFT NATURAL (English) and languages installed Portuguese (Brasil) and English (USA). For example typing ' then c , or any accent with the layout 'international with dead keys' leads to a character without accent. Ibus is activated, and deactivating it does not solve the issue.
Lucas Simon's solution worked perfectly in my Lubuntu 18.04.1 .
As I don't normally login as root, I opened an LXTerminal and ran the sudo variant of Lucas Simon's command line:
sudo apt install ibus && ibus-setup --xim
This quickly downloaded & installed some ibus packages, after which all my dead key accents immediately worked as intended - no further steps were needed, and I didn't even need to restart anything.