Add support for a content dir set per language

A sample config:

```toml
defaultContentLanguage = "en"
defaultContentLanguageInSubdir = true

[Languages]
[Languages.en]
weight = 10
title = "In English"
languageName = "English"
contentDir = "content/english"

[Languages.nn]
weight = 20
title = "På Norsk"
languageName = "Norsk"
contentDir = "content/norwegian"
```

The value of `contentDir` can be any valid path, even absolute path references. The only restriction is that the content dirs cannot overlap.

The content files will be assigned a language by

1. The placement: `content/norwegian/post/my-post.md` will be read as Norwegian content.
2. The filename: `content/english/post/my-post.nn.md` will be read as Norwegian even if it lives in the English content folder.

The content directories will be merged into a big virtual filesystem with one simple rule: The most specific language file will win.
This means that if both `content/norwegian/post/my-post.md` and `content/english/post/my-post.nn.md` exists, they will be considered duplicates and the version inside `content/norwegian` will win.

Note that translations will be automatically assigned by Hugo by the content file's relative placement, so `content/norwegian/post/my-post.md` will be a translation of `content/english/post/my-post.md`.

If this does not work for you, you can connect the translations together by setting a `translationKey` in the content files' front matter.

Fixes #4523
Fixes #4552
Fixes #4553

3 files changed, 23 insertions, 3 deletions

diff --git a/create/content.go b/create/content.go
index 8af417294..29fe47394 100644
--- a/create/content.go
+++ b/create/content.go
@@ -63,7 +63,22 @@ func NewContent(
 		return err
 	}
 
-	contentPath := s.PathSpec.AbsPathify(filepath.Join(s.Cfg.GetString("contentDir"), targetPath))
+	// The site may have multiple content dirs, and we currently do not know which contentDir the
+	// user wants to create this content in. We should improve on this, but we start by testing if the
+	// provided path points to an existing dir. If so, use it as is.
+	var contentPath string
+	var exists bool
+	targetDir := filepath.Dir(targetPath)
+
+	if targetDir != "" && targetDir != "." {
+		exists, _ = helpers.Exists(targetDir, ps.Fs.Source)
+	}
+
+	if exists {
+		contentPath = targetPath
+	} else {
+		contentPath = s.PathSpec.AbsPathify(filepath.Join(s.Cfg.GetString("contentDir"), targetPath))
+	}
 
 	if err := helpers.SafeWriteToDisk(contentPath, bytes.NewReader(content), s.Fs.Source); err != nil {
 		return err
diff --git a/create/content_template_handler.go b/create/content_template_handler.go
index 705efbd20..e9e7cb62b 100644
--- a/create/content_template_handler.go
+++ b/create/content_template_handler.go
@@ -88,10 +88,15 @@ func executeArcheTypeAsTemplate(s *hugolib.Site, kind, targetPath, archetypeFile
 		err               error
 	)
 
-	sp := source.NewSourceSpec(s.Deps.Cfg, s.Deps.Fs)
+	ps, err := helpers.NewPathSpec(s.Deps.Fs, s.Deps.Cfg)
+	sp := source.NewSourceSpec(ps, ps.Fs.Source)
+	if err != nil {
+		return nil, err
+	}
 	f := sp.NewFileInfo("", targetPath, false, nil)
 
 	name := f.TranslationBaseName()
+
 	if name == "index" || name == "_index" {
 		// Page bundles; the directory name will hopefully have a better name.
 		dir := strings.TrimSuffix(f.Dir(), helpers.FilePathSeparator)
diff --git a/create/content_test.go b/create/content_test.go
index 914759164..62d5ed1da 100644
--- a/create/content_test.go
+++ b/create/content_test.go
@@ -75,7 +75,7 @@ func TestNewContent(t *testing.T) {
 		for i, v := range c.expected {
 			found := strings.Contains(content, v)
 			if !found {
-				t.Errorf("[%d] %q missing from output:\n%q", i, v, content)
+				t.Fatalf("[%d] %q missing from output:\n%q", i, v, content)
 			}
 		}
 	}