The Configuration API allows for each value to be translated into each site language. These translations can be added in a few different ways.
Configuration and multilingual sites
Configuration forms
Any form that extends ConfigFormBase will display a tab that allows users to translate the configuration elements.
Translate configuration tab
Clicking on the tab shows a list of system languages with buttons to Add, Edit, and Delete translations for the form.
Language selection form
Selecting to Add or Edit the configuration in the language displays a form to allow translations. This form is built using the Configuration schema for the configuration names declared by the underlying configuration form.
Form translation
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