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/sync-translation-templates-to-locales.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+# This script updates the translation files (locales/<language>/**/*.po) for a given language.
+# It uses the translation templates (translation-templates/**/*.pot) to generate the translation files.
+
+set -euxo pipefail
+
+if [ $# -ne 1 ]; then
+    echo "Usage: $0 <language>"
+    exit 1
+fi
+
+LANGUAGE=$1
+
+base_path="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+
+if [ ! -f ${base_path}/i18n/translation-templates/README.pot ]; then
+    echo "Translation templates not found. Please run the `extract-translation-templates.sh` script first."
+    exit 1
+fi
+
+# Create necessary directories to avoid race conditions caused by
+# Sphinx potentially trying to concurrently create them from separate threads below.
+mkdir -p ${base_path}/i18n/locales/${LANGUAGE}/LC_MESSAGES/docs
+
+# Update the translation files
+sphinx-intl update \
+    --pot-dir ${base_path}/i18n/translation-templates \
+    --locale-dir ${base_path}/i18n/locales \
+    --language ${LANGUAGE}