Make string sorting (e.g. ByTitle, ByLinkTitle and ByParam) language aware

Fixes #2180

resources/page/integration_test.go

@@ -70,3 +70,69 @@ date: "2020-02-01"
 	b.AssertFileContent("public/en/index.html", "0|February, 2020|Pages(1)1|January, 2020|Pages(1)")
 	b.AssertFileContent("public/fr/index.html", "0|février, 2020|Pages(1)1|janvier, 2020|Pages(1)")
 }
+
+func TestPagesSortCollation(t *testing.T) {
+
+	files := `
+-- config.toml --
+defaultContentLanguage = 'en'
+defaultContentLanguageInSubdir = true
+[languages]
+[languages.en]
+title = 'My blog'
+weight = 1
+[languages.fr]
+title = 'Mon blogue'
+weight = 2
+[languages.nn]
+title = 'Bloggen min'
+weight = 3
+-- content/p1.md --
+---
+title: "zulu"
+date: "2020-01-01"
+param1: "xylophone"
+tags: ["xylophone", "éclair", "zulu", "emma"]
+---
+-- content/p2.md --
+---
+title: "émotion"
+date: "2020-01-01"
+param1: "violin"
+---
+-- content/p3.md --
+---
+title: "alpha"
+date: "2020-01-01"
+param1: "éclair"
+---
+-- layouts/index.html --
+ByTitle: {{ range site.RegularPages.ByTitle }}{{ .Title }}|{{ end }}
+ByLinkTitle: {{ range site.RegularPages.ByLinkTitle }}{{ .Title }}|{{ end }}
+ByParam: {{ range site.RegularPages.ByParam "param1" }}{{ .Params.param1 }}|{{ end }}
+Tags Alphabetical: {{  range site.Taxonomies.tags.Alphabetical }}{{ .Term }}|{{ end }}
+GroupBy: {{ range site.RegularPages.GroupBy "Title" }}{{ .Key }}|{{ end }}
+{{ with (site.GetPage "p1").Params.tags }}
+Sort: {{  sort . }}
+ByWeight: {{ range site.RegularPages.ByWeight }}{{ .Title }}|{{ end }}
+{{ end }}
+
+	`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+			NeedsOsFS:   true,
+		}).Build()
+
+	b.AssertFileContent("public/en/index.html", `
+ByTitle: alpha|émotion|zulu|
+ByLinkTitle: alpha|émotion|zulu|
+ByParam: éclair|violin|xylophone
+Tags Alphabetical: éclair|emma|xylophone|zulu|
+GroupBy: alpha|émotion|zulu|
+Sort: [éclair emma xylophone zulu]
+ByWeight: alpha|émotion|zulu|
+`)
+}