Why are country names in English?
If you have enabled and configured state and country picklists for your org, Salesforce and, consequently, JustOn fetch the corresponding values from the Salesforce database. By default, this results in English country names when printing addresses to PDF files.
Classic templates
This answer applies to classic templates. On a standard template, the addresses are mapped directly in the Liquid file from the invoice, account and tenant records – handle the country name there.
Usually, postal operators accept English country names as part of mailing addresses. So just using Salesforce's default feature is fine in most cases.