How lastmod in xml is generated?

I convert my website to a multi-language site just today but I find the lastmod property of my new added language is set to be roughly the first date of this website.
Does anybody know how hugo sets the lastmod in xml? Thanks.

I found the solution by myself.
It checks the latest date or lastmodified date in the front matter of all files. This latest date will be the lastmod in xml file.

I copied the front matter of the existing files to the files in newly-added languageā€¦

You may know this already, but if you have gitinfo enabled, you can also get the actual last modified date of the file, without having to use front matter.