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, 51 insertions, 0 deletions

diff --git a/hugofs/language_composite_fs.go b/hugofs/language_composite_fs.go
new file mode 100644
index 000000000..2889f8a00
--- /dev/null
+++ b/hugofs/language_composite_fs.go
@@ -0,0 +1,51 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugofs
+
+import (
+	"github.com/spf13/afero"
+)
+
+var (
+	_ afero.Fs      = (*languageCompositeFs)(nil)
+	_ afero.Lstater = (*languageCompositeFs)(nil)
+)
+
+type languageCompositeFs struct {
+	*afero.CopyOnWriteFs
+}
+
+// NewLanguageCompositeFs creates a composite and language aware filesystem.
+// This is a hybrid filesystem. To get a specific file in Open, Stat etc., use the full filename
+// to the target filesystem. This information is available in Readdir, Stat etc. via the
+// special LanguageFileInfo FileInfo implementation.
+func NewLanguageCompositeFs(base afero.Fs, overlay *LanguageFs) afero.Fs {
+	return afero.NewReadOnlyFs(&languageCompositeFs{afero.NewCopyOnWriteFs(base, overlay).(*afero.CopyOnWriteFs)})
+}
+
+// Open takes the full path to the file in the target filesystem. If it is a directory, it gets merged
+// using the language as a weight.
+func (fs *languageCompositeFs) Open(name string) (afero.File, error) {
+	f, err := fs.CopyOnWriteFs.Open(name)
+	if err != nil {
+		return nil, err
+	}
+
+	fu, ok := f.(*afero.UnionFile)
+	if ok {
+		// This is a directory: Merge it.
+		fu.Merger = LanguageDirsMerger
+	}
+	return f, nil
+}