resources/page: Deprecate .Sites.First in favor of .Sites.Default

Closes #12513

resources/page/site_integration_test.go

@@ -0,0 +1,44 @@
+// Copyright 2024 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package page_test
+
+import (
+	"testing"
+
+	"github.com/gohugoio/hugo/hugolib"
+)
+
+// Issue 12513
+func TestPageSiteSitesDefault(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['page','rss','section','sitemap','taxonomy','term']
+defaultContentLanguage = 'de'
+defaultContentLanguageInSubdir = true
+[languages.en]
+languageName = 'English'
+weight = 1
+[languages.de]
+languageName = 'Deutsch'
+weight = 2
+-- layouts/index.html --
+{{ .Site.Sites.Default.Language.LanguageName }}
+`
+
+	b := hugolib.Test(t, files)
+
+	b.AssertFileContent("public/de/index.html", "Deutsch")
+}