github: Add weekly pull request from Weblate translations.

Author: alexmv

.github/workflows/update-translations.yml

@@ -0,0 +1,21 @@
+name: Update translations from Weblate
+on:
+  schedule:
+    - cron: "0 10 * * 1"
+  workflow_dispatch:
+jobs:
+  update-translations:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Fetch and merge from Weblate
+        # The commit message is generated in Weblate; see https://hosted.weblate.org/addon/17163/
+        run: |
+          git remote add weblate https://hosted.weblate.org/git/zulip/zulip-flutter/
+          git merge --ff-only weblate/main
+      - name: Create Pull Request
+        uses: peter-evans/create-pull-request@v7
+        with:
+          branch: update-translations/weblate
+          delete-branch: true
+          title: Update translations from Weblate