@erniecom maybe you need to set the site language to the preferred language first? I’ve never had the need to do something like that, so, don’t really know. Seems like it would depend on whether PHP itself is multilingual with their error messages.
If you refer to the site language as per WordPress then that in my case is not allowed. The site owner requires the default to be Spanish.
I am not sure if the debug messages come straight from PHP. Some do and are in English. Others are written in the default language of the site. The idea I am after here is that these messages will in English too. One of the obvious reasons is that I would like to search for them in one language.
@erniecom error messages most probably come straight out of PHP, most of the time, unless plugins/themes has their own error handling methods which outputs error messages that are internationalized and have been translated / have translations included in the plugins/themes.
https://www.google.com/search?q=wordpress+debug+log+multilingual yields nothing of interest and https://www.php.net/manual/en/function.error-reporting.php makes no mention of language selection, so, I assume error messages are only available in English.
However, do let me know if you find that it’s not the case and there is a way to access non-English version of error messages in PHP and/or WordPress. I’d be interested to know as well.