Keyboard Layout: by this option plasma users are able to change their favorite keyboard layouts in any type of language according to following picture.
This option is accessible by Programs Menu > Settings > System Settings > Input Devices > Keyboard settings > Layouts > Add
Language Setting: if anytime u decided to change the entire workspace language then you just need to check the following address:
Programs Menu > Settings > System Settings > Local > Country / Region & Language > Languages
If you couldn't find your favorite language in the list the you may need to install the necessary language files from package manager or simply Apper by Programs Menu > System > Apper (Software Management)