Add a way to disable one or more languages

This commit adds a new config setting:

```toml
disableLanguages = ["fr"]
```

If this is a multilingual site:

* No site for the French language will be created
* French content pages will be ignored/not read
* The French language configuration (menus etc.) will also be ignored

This makes it possible to start translating new languages and turn it on when you're happy etc.

Fixes #4297
Fixed #4329

hugolib/fileInfo.go

@@ -31,6 +31,9 @@ type fileInfo struct {
 	bundleTp bundleDirType
 	source.ReadableFile
 	overriddenLang string
+
+	// Set if the content language for this file is disabled.
+	disabled bool
 }
 
 func (fi *fileInfo) Lang() string {
@@ -60,6 +63,9 @@ func newFileInfo(sp *source.SourceSpec, baseDir, filename string, fi os.FileInfo
 		ReadableFile: baseFi,
 	}
 
+	lang := f.Lang()
+	f.disabled = lang != "" && sp.DisabledLanguages[lang]
+
 	return f
 
 }