To translate the letter, copy index.md into _translations/
and name it index_lang.md where lang is language code you are going to translate into.
Make sure to set locale to the appropriate language code. Set image to a translated version of social-media-preview.png (for an editable template, see #3324), if there's any. Then you can start translating this file.
For example: _translations/index_de.md
---
layout: signed
title: An open letter in support of Richard M. Stallman
description: An open letter in support of Richard Matthew Stallman being reinstated by the Free Software Foundation
image: /assets/social-media-preview.png
locale: de_DE
twitter:
card: summary_large_image
---
2021-03-23
Richard M. Stallman, ...There are lots of translation already, if you feel lost, be sure to check out how its done in other ones.
When you translated the text you can commit your change and make pull request.