So you will have to install the language pack as well besides following the steps.
Language packs can be found for example here.
I just copy and paste the installation instructions:
Download file corresponding to your language.
It is a single file, that contains translations and module context. Easiest to download it to your installation root directory.
Now run the following command:
bin/magento i18n:pack -m replace -d source_[lang_code].csv langpack [lang_code]
where [lang_code] is language code like pl_PL.
As a result, a directory langpack will be created that contains proper structure of language files for your installation.
Copy it over to your Magento installation and the translations should be visible. If not, run the following commands to clear cache:
php bin/magento setup:static-content:deploy [lang_code]
php bin/magento indexer:reindex
php bin/magento cache:clean
php bin/magento cache:flush
Note: additional directory i18n contains strings for which the module could not be found in your installation. This file can be skipped.
Another useful source here
Hope that helps you