You should upload the content of the "public_html" directory from your downloaded translations to the root directory where Blesta is installed on your server.
For example:
Blesta is installed at "manage.mydomain.com", but the file structure is "/home/user/username/public_html/manage/"
Upload the files to the "/home/user/username/public_html/manage/" directory
Upload what is _inside_ the "public_html" directory from your downloaded translations, not the "public_html" directory itself