Allow setting locale in frontmatter

[IanVS]

Is your feature request related to a problem? Please describe.
I am using astro's dynamic routing to build up my urls, and it would be really nice if I could add a hidden locale field to my collection so that it appears in the frontmatter. As it stands, it seems the only way to determine a file's locale is from the path/filename, unless I'm missing another option.

Describe the solution you'd like
Ideally, this would be as easy as adding a field like { name: "locale", label: "Locale", widget: "hidden", default: "{{locale}}", i18n: true},

Describe alternatives you've considered
I tried to create a preSave handler, but it seems to only run once per entry, and expects data to be returned, not the i18n map which contains the data for the localized versions.

[imskr]

I am working on this @erezrokah