Whenever I change language layout, this notification appears and after 3 seconds disappear and I do not want to see this notification.
My solution for 18.04 LTS. As a Thai people, we have a norm to use the ` key to switch between en/th. So I try it too, and found out that when I chose (pressed) ` key in the settings 'Switch to the next input source', the system automatically choose Shift + ` in 'Switch to the previous input source' for me. And with this settings, when I pressed only ` ..I got a pop-up too. But when I try hold-pressed Shift key and then pressed ` ..NOW it's has no pop-up!. This more safety than the default 'SuperSpace(press these in simultaneously)' that can sometime-freezed the desktop. (But if you have to or want to use
~ (both are on the same key) you can still copy it from characters map.)
That language indicating overlay is part of the GNOME shell, so probably not so easy to switch it off completely.
As a workaround you may set another alternative keyboard shortcut to switch between input languages, for example Alt+Shift which will not show the overlay. To do so, follow the Q&A: Ubuntu 17.10 can't change the input switching shortcut to alt+shift.