Hope you are doing well. Make sure you have data connection or wifi connection to download language. Have you tried this steps:
From the Home screen, swipe up, then tap Settings.
Tap General management > Language and input > Language.
Tap + Add language.
Select from the list of available languages to choose from.
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Resolution that worked
To show the downloadable languages you need to clear both Data and Cache in Samsung Keyboard App.
Settings>Apps>Samsung Keybord>Storage>Clear data
Settings>Apps>Samsung Keybord>Storage>Clear cache
Accessing keyboard from any text area will invoke data update and will show the complete list of the downloadable languages to select the perferred language(s).