Add GitHub Action for checking PO files

.github/workflows/check-translations.yml

@@ -0,0 +1,27 @@
+name: Check translations
+
+on:
+  push:
+    paths:
+    - 'locales/*.po'
+  pull_request:
+    paths:
+    - 'locales/*.po'
+
+jobs:
+  check-po-validity:
+    name: Check translations
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Install GNU gettext
+      run: sudo apt-get install gettext
+
+    - name: Check PO files with msgfmt -v -c
+      run: |
+        for i in translations/*.po ; do
+          echo "checking $i..."
+          msgfmt -v -c -o /dev/null $i
+        done