Initial work on translations / localization

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3841

Most of the preparation for this was done by Suguru Hirahara (https://github.com/luixxiul).
I've merely reorganized/polished the scripts and instructions in the `i18n/` directory.

While translations can happen even now, more work is necessary to

- make the translation flow better (integrating Weblate), etc.

- restore the Github Actions workflows that Suguru Hirahara had already developed to
  adapt them to our new workflow

i18n/bin/extract-translation-templates.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+
+# This script extracts translation templates (original English strings) into the `translation-templates/` directory.
+# These templates are later used to generate locale files for each language in the `locales/` directory.
+#
+# By default `sphinx-build` extracts the templates into a `build/gettext` directory, while we'd like to have them in the `translation-templates/` directory.
+# To avoid the `POT-Creation-Date` information in templates being updated every time we extract them,
+# we restore the `translation-templates/` directory to the `build/gettext` directory before running the script.
+
+set -euxo pipefail
+
+base_path="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+
+# Restore the `translation-templates/` directory to the `build/gettext` directory
+if [ -d ${base_path}/i18n/build ]; then
+    rm -rf ${base_path}/i18n/build
+fi
+mkdir -p ${base_path}/i18n/build
+cp -r ${base_path}/i18n/translation-templates ${base_path}/i18n/build/gettext
+
+# Extract translation templates from the documentation into the `build/gettext` directory
+sphinx-build -M gettext ${base_path} ${base_path}/i18n/build
+
+# Clean up the build directory
+rm -rf ${base_path}/i18n/build/gettext/.doctrees
+
+# Update the `translation-templates/` directory with the new templates
+if [ -d ${base_path}/i18n/translation-templates ]; then
+    rm -rf ${base_path}/i18n/translation-templates
+fi
+mv ${base_path}/i18n/build/gettext ${base_path}/i18n/translation-templates
+
+# Get rid of the `build` directory
+rmdir ${base_path}/i18n/build