I was using Ubuntu 16.04. There it was so easy to change the text entry option. Settings > Text entry.
But when I switch to Ubuntu 18.04 I can't find such option. So how can I write in another language? How to change text entry i.e. by super+shift or something like that?
How to add and select a language in Ubuntu 18.04 LTS
Edited to make it easier to find your language. Thanks to @Gunnar Hjalmarsson.
Click near the top right corner, select the 'tools' symbol
You arrive at the Settings menu. Select Region & Language
Click on the long button Manage Installed Languages
If the system is new, a window may pop up: 'The language support is not installed completely'. In that case click on Install
Click on Install/Remove Languages
Select and tick your language(s), like I did with Swedish
Click on Apply
Now you might want to select language for menus and windows and drag it to the top of the box
It should look like this with your language at the top of the box
Now you might want to click on Apply System-Wide
Close the window 'Language support' and the Settings menu
Click near the top right corner, select the 'tools' symbol to restart the Settings menu
Back in the Settings menu, Click on +
Mark 'your language' (that you should find directly without clicking on
⋮
)Click on the green button Add
Mark your language in the list
Click on ^ to move it to the top of the list
Now you can select between English and your language. Click on the applet near the top right corner of the screen.
How to assign a keyboard shortcut to switch keyboard layout
The default shortcuts for keyboard layout switching are Super+Space and Shift+Super+Space. @N0rbert suggested to add a screenshot with assigning of keyboard shortcut to keyboard layout switching. It is a good idea, and there is a detailed description at this link,
18.04 ctrl+shift to change language
Right-click and open the screenshot picture in another [big] window to see it in full size. That way it is easier to read the text.