Add a way to merge pages by language

As an example:

```html
{{ $pages := .Site.RegularPages | lang.Merge $frSite.RegularPages | lang.Merge $enSite.RegularPages }}
```

Will "fill in the gaps" in the current site with, from left to right, content from the French site, and lastly the English.

Fixes #4463

tpl/lang/lang.go

@@ -15,6 +15,7 @@ package lang
 
 import (
 	"errors"
+	"fmt"
 	"math"
 	"strconv"
 	"strings"
@@ -134,3 +135,15 @@ func (ns *Namespace) NumFmt(precision, number interface{}, options ...interface{
 
 	return string(b), nil
 }
+
+type pagesLanguageMerger interface {
+	MergeByLanguageInterface(other interface{}) (interface{}, error)
+}
+
+func (ns *Namespace) Merge(p2, p1 interface{}) (interface{}, error) {
+	merger, ok := p1.(pagesLanguageMerger)
+	if !ok {
+		return nil, fmt.Errorf("language merge not supported for %T", p1)
+	}
+	return merger.MergeByLanguageInterface(p2)
+}