Instead of changing the Japanese language files, you can duplicate them and then change to Chinese. This would provide all included languages and the additional language you wish to add.
See this GitHub commit for ideas on all files that need changing and the config/languages.php file that needs the addition of your added language.
https://github.com/phpvms/phpvms/pull/1890/commits/34d7fe417dd979b42050e2f6597e2b47ff582dcd
Once you are done, consider submitting a PR to the phpVMS GitHub repo for possible future inclusion.