When I visit another page such as hidden link for example it looks like it's working as expected but the homepage is throwing the certificate error.
- Verify that in your directory(you can do this through FTP manager or the File Manager in Cpanel) there are no physical directories
- Are you currently using any SSL plugins? What are your results with Really Simple SSL ?
Typically when using different directories per language as the structure there should not be any issues or anything extra steps to be done for the SSL certificate, I would advise reaching out to your hosting provider as well.
Let mee know of your results and their feedback, please.