Can't use Polish language diacritic characters in Ubuntu 18.04

Question

I use Ubuntu 18.04

When I press right Alt + any diacritic letter, be it a, c or s I get nothing. Same goes for using left Alt+ any letter.

My input source is set to Polish.

Why can't I type in Polish?

Answer 1

In GNOME Tweaks I set Keyboard & Mouse->Additional Layout Options -> Key to choose the 3rd level -> Right Alt.

Now Polish diacritic characters work as expected.