Is your feature request related to a problem? Please describe.

I'm trying to find a way to get netlify-cms to understand French/English page pairs in different subfolders. I'm working with Hugo, and the way they handle translation of paths is to provide a translationKey frontmatter field that links two pages in differently named folders.

Given the following file structure:

src
├── content
│   ├── en
│   │   └── resources
│   │       └── index.md
│   └── fr
│       └── ressources
│           └── index.md

with frontmatter for the en one like:

---
title: Resources
slug: resources
date: 2020-10-30T14:33:09.744Z
translationKey: resources
---

vs the fr one:

---
title: Ressources
slug: ressources
date: 2020-10-30T14:33:09.744Z
translationKey: resources
---

and the following netlify-cms config.yml:

.
.
.
i18n:
  structure: multiple_folders
  locales: [en, fr]
collections:
  - name: 'page'
    label: 'page'
    folder: '/src/content'
    i18n: true
    create: true
    editor:
      preview: true
    nested:
      depth: 100 # max depth to show in the collection tree
      summary: '{{title}}' # optional summary for a tree node, defaults to the inferred title field
    fields:
      - { label: 'Title', name: 'title', i18n: true, widget: 'string' }
      - { label: 'Path', name: 'slug', i18n: true, widget: 'string', required: false }
      - { label: 'Meta description', name: 'description', required: false, i18n: true, widget: 'string' }
      - { label: 'Publish Date', name: 'date', i18n: true, widget: 'datetime' }
      - { label: 'Translation key', name: 'translationKey', required: false, i18n: true, widget: 'hidden', default: '{{slug}}' }
      - { label: 'Body', name: 'body', i18n: true, widget: 'markdown' }

Netlify-cms sees them as unrelated pages:

and

Likewise when creating content. I fill out the English and French sides of the editor, adding different path values, and get pages with this kind of frontmatter:

src/content/en/covid-19.md

---
title: Covid-19
slug: covid
description: ""
date: 2020-11-27T14:44:25.843Z
translationKey: "{{slug}}"
---

and
src/content/en/covid-19.md

---
title: Covid-19
slug: covid
date: 2020-11-27T14:44:25.859Z
translationKey: "{{slug}}"
---

(not sure why description doesn't show up in the French file, but that's another issue).

The {{slug}} part isn't being resolved as expected. I'd expect to see covid there.

Describe the solution you'd like

I'd like to see 2 things:

1. The collections browser should use frontmatter like translationKey (for Hugo; not sure what equivalents are for other frameworks) to match translated files.
2. The widgets like string and hidden should support parsing template tags like {{slug}}.