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

hugolib/config.go

@@ -130,21 +130,17 @@ func loadLanguageSettings(cfg config.Provider, oldLangs helpers.Languages) error
 	} else {
 		languages = make(map[string]interface{})
 		for k, v := range languagesFromConfig {
-			isDisabled := false
 			for _, disabled := range disableLanguages {
 				if disabled == defaultLang {
 					return fmt.Errorf("cannot disable default language %q", defaultLang)
 				}
 
 				if strings.EqualFold(k, disabled) {
-					isDisabled = true
+					v.(map[string]interface{})["disabled"] = true
 					break
 				}
 			}
-			if !isDisabled {
-				languages[k] = v
-			}
-
+			languages[k] = v
 		}
 	}