ci: Check POTFILES in CI

We already do this in xdg-desktop-portal and it would have caught
missing files in POTFILES.in already.

.github/workflows/check-potfiles.sh

@@ -0,0 +1,92 @@
+#!/usr/bin/env bash
+
+# FIXME stolen from gnome-control-center, GPLv2
+# This project is LGPLv2
+# https://gitlab.gnome.org/GNOME/gnome-control-center/-/merge_requests/2269#note_2079291
+
+srcdirs=(app common oci-authenticator portal system-helper)
+uidirs=( "${srcdirs[@]}" )
+desktopdirs=( "${srcdirs[@]}" )
+policydirs=( "${srcdirs[@]}" )
+xmldirs=( "${srcdirs[@]}" )
+
+# find source files that contain gettext functions with literal or GETTEXT_PACKAGE argument
+files=$(grep -lRs --include='*.c' 'gettext *(\("\|GETTEXT_PACKAGE,\)' "${srcdirs[@]}")
+
+# find source files that contain gettext macros
+files="$files "$(grep -lRs --include='*.c' --include='*.h' '[^I_)]_(' "${srcdirs[@]}")
+
+# find ui files that contain translatable string
+files="$files "$(grep -lRis --include='*.ui' 'translatable="[ty1]' "${uidirs[@]}")
+
+# find .desktop files
+files="$files "$(find "${desktopdirs[@]}" -name '*.desktop*')
+
+# find .policy files
+files="$files "$(find "${policydirs[@]}" -name '*.policy*')
+
+# find .xml.in and .gschema.xml files
+files="$files "$(find "${xmldirs[@]}" -not -name '*.gresource.xml*' \
+                               -name '*.xml.in*' -o \
+                               -name '*.gschema.xml')
+
+# filter out excluded files
+if [ -f po/POTFILES.skip ]; then
+  files=$(for f in $files; do
+            ! grep -q "^$f" po/POTFILES.skip && echo "$f"
+          done)
+fi
+
+# find those that aren't listed in POTFILES.in
+missing=$(for f in $files; do
+            ! grep -q "^$f" po/POTFILES.in && echo "$f"
+          done)
+
+# find those that are listed in POTFILES.in but do not contain translatable strings
+invalid=$(while IFS= read -r f; do
+            ! grep -q "$f" <<< "$files" && echo "$f"
+          done < <(grep '^[^#]' po/POTFILES.in))
+
+# find out if POTFILES.in is sorted correctly, ignoring empty lines
+sorted=$(grep '^[^#]' po/POTFILES.in | \
+         LC_ALL=en_US.UTF-8 sort -cu 2>&1 >/dev/null | \
+         awk -F ': *' '{print $5}')
+
+if [ ${#missing} -eq 0 ] && [ ${#invalid} -eq 0 ] && [ ${#sorted} -eq 0 ]; then
+  exit 0
+fi
+
+if [ ${#missing} -ne 0 ]; then
+  cat >&2 << EOT
+
+The following files are missing from po/POTFILES.in or po/POTFILES.skip:
+
+EOT
+  for f in $missing; do
+    echo "  $f" >&2
+  done
+fi
+
+if [ ${#invalid} -ne 0 ]; then
+  cat >&2 << EOT
+
+The following files are in po/POTFILES.in but are missing or not translatable:
+
+EOT
+  for f in $invalid; do
+    echo "  $f" >&2
+  done
+fi
+
+if [ ${#sorted} -ne 0 ]; then
+  cat >&2 << EOT
+
+The following file is not sorted properly in po/POTFILES.in:
+
+EOT
+  echo "  $sorted" >&2
+fi
+
+echo >&2
+
+exit 1

.github/workflows/check-potfiles.yml

@@ -0,0 +1,28 @@
+name: Check POTFILES.in
+
+on:
+  push:
+    branches:
+    - main
+    - 'flatpak-1.[0-9]+.x'
+  pull_request:
+    branches:
+    - main
+    - 'flatpak-1.[0-9]+.x'
+  merge_group:
+    types:
+    - checks_requested
+
+jobs:
+  potfiles:
+    name: Check POTFILES.in
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Repository checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Check POTFILES.in
+        run: .github/workflows/check-potfiles.sh