A "build" wouldn't change the language of the input echo, "language" is only an option during the installation. There is not a way to switch languages, other than to uninstall and then reinstall the software.
(Your issue is that all of the "*.inp" files in \caesarii510\system are in Spanish. If you can find another installation of 5.10 where the input echo is in English, you can just copy all of the INP files to this computer, and avoid the uninstall/reinstall steps.)
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Regards,
Richard Ay - Consultant