If you work with contributors who are not native in the language your website is set, those users might be more comfortable working in WordPress with their own language.
Note: the setting is not visible if the site language is set to the default en_US
To make it available, you need to set your site’s language to a different language first, so WordPress can pull the language files.
Any of the languages you set in the Settings > General will become available for the users once you set it there.
Here is a quote from the Make-Thread of this feature
“Never mind – found it. On a clean install, the “Language” field will not be displayed. You need to change the admin language for each language you want so WordPress will get the mo files. In a sense, users cannot simply pick a language, it must be one of the languages allowed by the site (so if the admin as only switched between 2 languages, you only get 2 languages).”
Install language packages for your WordPress site
Go to “Settings > General” scroll all the way down the page and use the “Language” drop-down. It shows you a list of installed languages as well as a list of available languages. Select one you need for your users and “Save Changes” –
You can certainly switch back the default language after that. And you find the new languages in the list under “Installed”
Installed languages are available for User Profiles
The installed languages from the “Setting > General” Screens are now available for your users via “Your Profile” screens.