[i18n] Initial multilingual support for redirects with ja example (#4687

)

.htmltest.yml

@@ -72,3 +72,5 @@ IgnoreURLs: # list of regexs of paths or URLs to be ignored
   - ^https://wikipedia.org/wiki/(S.M.A.R.T|Hop_)
   # TODO move into content/en/blog/2023/contributing-to-otel/index.md once https://github.com/open-telemetry/opentelemetry.io/issues/3889 is implemented
   - ^https://shorturl.at/vLYZ0$
+  # TODO remove the following temporary ignore rule (@chalin)
+  - ^/en/docs/specs$

content/ja/docs/specs/_index.md

@@ -0,0 +1,12 @@
+---
+title: Specifications
+linkTitle: Specs ↗
+description: _Redirect page_
+weight: 960
+# _build: { render: link }
+redirect: /en/docs/specs 301!
+redirects: [{ from: '*', to: '/en/docs/specs/:splat' }]
+default_lang_commit: 3b44fbfa49ced919daea01123abfaed836d2d0ec
+---
+
+Netlify redirect target: [{{% param "title" %}}]({{% param "redirect" %}}).

layouts/index.redirects

@@ -72,3 +72,7 @@
   {{ end }}
   {{ return $result }}
 {{ end }}
+
+{{/* Multilingual support */ -}}
+
+{{ partial "redirects/sites.redirects" . | partial "func/trim-lines.html" -}}

layouts/partials/func/trim-lines.html

@@ -0,0 +1,3 @@
+{{ range split . "\n" -}}
+{{ trim . " \t" }}
+{{ end -}}

layouts/partials/redirects/sites.redirects

@@ -0,0 +1,41 @@
+{{/* Redirect for default language when .LanguagePrefix is empty. */ -}}
+
+{{ $defaultLang := "" -}}
+{{ with .Site.Sites.Default -}}
+  {{ if eq .LanguagePrefix "" -}}
+    {{ with .Language.Lang -}}
+      {{ $defaultLang = . -}}
+      /{{ . }}    /
+      /{{ . }}/*  /:splat
+    {{ end -}}
+  {{ end -}}
+{{ end -}}
+
+{{/* Process non-default languages. */ -}}
+
+/ja/docs/specs  /en/docs/specs/ 301!
+
+{{ range after 1 .Sites -}}
+
+  {{ $siteLang := .Language.Lang -}}
+
+  # Site localization {{ $siteLang }}
+  {{ range $p := .Pages -}}
+
+    {{ range $p.Params.redirects -}}
+      {{ $fallbackPage := partial "i18n/fallback-page.html" $p -}}
+      {{ if or (eq $siteLang $defaultLang) (not $fallbackPage) -}}
+        {{ $from := cond (strings.HasPrefix .from "/")
+            .from
+            (print $p.RelPermalink .from) -}}
+        {{ $to := cond (strings.HasPrefix .to "/")
+            .to
+            (print $p.RelPermalink .to) -}}
+        {{ $from | printf "%-35s" }} {{ $to }}
+      {{ else -}}
+        {{/* # {{ $p.RelPermalink }} */ -}}
+      {{ end -}}
+    {{ end -}}
+  {{ end -}}
+
+{{ end -}}