How to change hot key to change language on Ctrl+Shift in Ubuntu 18.04?

You need to install gnome-tweaks:
sudo apt-get install gnome-tweak-tool
Then call gnome-tweaks, the left side choose "Keyboard and mouse".
Under the "shortcut ..." button the "more layout options" and click in the window choose the "Switching to another layout". There you can choose any combination from Alt+Shift, etc.

Comrades, killed plenty of time but found how to win :-) the Internet is full of information how to use gnome-tweak-tool to enable switching of keyboard layout by Alt+Shift, but we have something you need it to Alt+Shift worked switching input sources, and is another feature that is very convenient because it allows you to quickly switch between the last two languages, while there may be a footcloth of 5 languages (like me). The solution here, I have to warn you that there was no conflict, if you turn on Alt+Shift using gnome-tweak-tool, then disconnect there.

The bottom line:

1. To install the package dconf-editor and edit one or both of the following registry key to make it work:

2. Go to org.gnome.desktop.wm.keybindings

3. Change the input source switch on ['Shift_L']

4. Similarly, key switch-input-source-backward will change to ['Alt_L']

I used to press the buttons in reverse order, so did the turnover and running :-)