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

1 files changed, 2 insertions, 2 deletions

diff --git a/hugolib/menu_test.go b/hugolib/menu_test.go
index 357499526..6a8c89b95 100644
--- a/hugolib/menu_test.go
+++ b/hugolib/menu_test.go
@@ -74,8 +74,8 @@ Menu Main:  {{ partial "menu.html" (dict "page" . "menu" "main") }}`,
 	writeSource(t, fs, "content/sect2/p4.md", fmt.Sprintf(menuPageTemplate, "p4", 4, "main", "atitle4", 10))
 	writeSource(t, fs, "content/sect3/p5.md", fmt.Sprintf(menuPageTemplate, "p5", 5, "main", "atitle5", 5))
 
-	writeNewContentFile(t, fs, "Section One", "2017-01-01", "content/sect1/_index.md", 100)
-	writeNewContentFile(t, fs, "Section Five", "2017-01-01", "content/sect5/_index.md", 10)
+	writeNewContentFile(t, fs.Source, "Section One", "2017-01-01", "content/sect1/_index.md", 100)
+	writeNewContentFile(t, fs.Source, "Section Five", "2017-01-01", "content/sect5/_index.md", 10)
 
 	err := h.Build(BuildCfg{})