i18n Translations allows you to search all translations in all languages installed on your Jira instance.
But why? …
Because in many places of Jira, for example in Workflow Properties - Transition titles and buttons (jira.i18n.submit or jira.i18n.title), but also in add-ons you can use i18n to try add your own translations if the vendor did not foresee them.
This this functionality:
search for all translations, even those not visible (e.g. translations in emails, etc.) and cannot be searched using InProduct translation for Jira
does not allow adding new translations
If you want to read more about i18n go here https://developer.atlassian.com/server/framework/atlassian-sdk/internationalising-your-plugin/