ci: Add a check for potfiles

Use Florian's script in gnome-shell as basis and adapt it to our needs

.gitlab-ci.yml

@@ -25,6 +25,10 @@ style-check:
     paths:
       - "style-check-junit-report.xml"
 
+i18n:
+  stage: review
+  script:
+    - ./.gitlab/ci/check-potfiles.sh
 
 flatpak:
   image: quay.io/gnome_infrastructure/gnome-runtime-images:gnome-master
@@ -39,4 +43,4 @@ nightly:
 
 nightly aarch64:
   extends: '.publish_nightly'
-  needs: ['flatpak@aarch64']
\ No newline at end of file
+  needs: ['flatpak@aarch64']

.gitlab/ci/check-potfiles.sh

+#!/usr/bin/env bash
+
+srcdirs="src"
+uidirs="data/ui"
+
+# find source files that contain gettext keywords
+vala_files="$(grep -lR --include='*.vala' '\(gettext\|[^I_)]_\)(' $srcdirs)"
+c_files="$(grep -lR --include='*.c' '\(gettext\|[^I_)]_\)(' $srcdirs)"
+
+# find ui files that contain translatable string
+ui_files="$(grep -lRi --include='*.ui' 'translatable="[ty1]' $uidirs)"
+
+files="$vala_files $c_files $ui_files"
+
+# filter out excluded files
+if [ -f po/POTFILES.skip ]; then
+  files="$(for f in $files; do ! grep -q "^$f$" po/POTFILES.skip && echo "$f"; done)"
+fi
+
+# Test 1: find all files that are missing from POTFILES.in
+missing="$(for f in $files; do ! grep -q "^$f$" po/POTFILES.in && echo "$f"; done)"
+if [ ${#missing} -ne 0 ]; then
+  echo >&2 "The following files are missing from po/POTFILES.in:"
+  for f in ${missing[@]}; do
+    echo "  $f" >&2
+  done
+  echo >&2
+  exit 1
+fi
+
+# Test 2: find all Vala files that miss a corresponding .c file in POTFILES.skip
+vala_c_files="$(for f in $vala_files; do echo "${f%.vala}.c"; done)"
+vala_c_files_missing="$(for f in ${vala_c_files[@]}; do ! grep -q "^$f$" po/POTFILES.skip && echo "$f"; done)"
+if [ ${#vala_c_files_missing} -ne 0 ]; then
+  echo >&2 "The following files are missing from po/POTFILES.skip:"
+  for f in ${vala_c_files_missing[@]}; do
+    echo "  $f" >&2
+  done
+  echo >&2
+  exit 1
+fi