The navigation to change the keyboard input language in vanilla Ubuntu is Settings -> Keyboard → Input Sources -> + instead of Settings -> Region & Language -> Input Sources -> Manage Installed Languages -> + Click the three vertical dots ⋮ (vertical ellipsis) to show the rest of the input languages.
Depending on your Ubuntu version it may be either Region & Language as shown in your screenshot or Keyboard as it is in later versions of Ubuntu (just in case you upgrade to a later version of Ubuntu).
Manage Installed Languages is optional. Only click this if you want to install translation packs for the chosen new input language.
The navigation to change the keyboard input language in vanilla Ubuntu is Settings -> Keyboard → Input Sources -> + instead of Settings -> Region & Language -> Input Sources -> Manage Installed Languages -> + Click the three vertical dots ⋮ (vertical ellipsis) to show the rest of the input languages.
Depending on your Ubuntu version it may be either Region & Language as shown in your screenshot or Keyboard as it is in later versions of Ubuntu (just in case you upgrade to a later version of Ubuntu).
Manage Installed Languages is optional. Only click this if you want to install translation packs for the chosen new input language.