In Ubuntu 16.04 (which used the Unity desktop) I have a language selector in the top bar, next to the network icon and messages icon. Is there a way to have something similar in Ubuntu 19.04 (which uses the GNOME desktop)? (Desktop differences more)? I switch languages frequently, like when I want to type one word in Greek.
You need to set up multiple input languages (keyboard layouts) first. The input language switcher icon would appear automatically in the top bar once you have two or more input languages.
Also as pointed out by @singrium in comments, you can alternatively use the Super+Space keyboard shortcut to switch between input languages.