Fix sectionPagesMenu for pages in root level

Fixes #12306

hugolib/menu_test.go

@@ -636,3 +636,43 @@ Menu Item: {{ $i }}|{{ .URL }}|
 Menu Item: 0|/foo/posts|
 `)
 }
+
+func TestSectionPagesMenuMultilingualWarningIssue12306(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['section','rss','sitemap','taxonomy','term']
+defaultContentLanguageInSubdir = true
+sectionPagesMenu = "main"
+[languages.en]
+[languages.fr]
+-- layouts/_default/home.html --
+{{- range site.Menus.main -}}
+  <a href="{{ .URL }}">{{ .Name }}</a>
+{{- end -}}
+-- layouts/_default/single.html --
+{{ .Title }}
+-- content/p1.en.md --
+---
+title: p1
+menu: main
+---
+-- content/p1.fr.md --
+---
+title: p1
+menu: main
+---
+-- content/p2.en.md --
+---
+title: p2
+menu: main
+---
+`
+
+	b := Test(t, files, TestOptWarn())
+
+	b.AssertFileContent("public/en/index.html", `<a href="/en/p1/">p1</a><a href="/en/p2/">p2</a>`)
+	b.AssertFileContent("public/fr/index.html", `<a href="/fr/p1/">p1</a>`)
+	b.AssertLogNotContains("WARN")
+}

hugolib/site.go

@@ -658,8 +658,13 @@ func (s *Site) assembleMenus() error {
 			if p.IsHome() || !p.m.shouldBeCheckedForMenuDefinitions() {
 				return false, nil
 			}
+
 			// The section pages menus are attached to the top level section.
 			id := p.Section()
+			if id == "" {
+				id = "/"
+			}
+
 			if _, ok := flat[twoD{sectionPagesMenu, id}]; ok {
 				return false, nil
 			}
@@ -671,13 +676,15 @@ func (s *Site) assembleMenus() error {
 				},
 				Page: p,
 			}
+
 			navigation.SetPageValues(&me, p)
 			flat[twoD{sectionPagesMenu, me.KeyName()}] = &me
 			return false, nil
 		}); err != nil {
 			return err
 		}
 	}
+
 	// Add menu entries provided by pages
 	if err := s.pageMap.forEachPage(pagePredicates.ShouldListGlobal, func(p *pageState) (bool, error) {
 		for name, me := range p.pageMenus.menus() {