hugolib: Make sure everything ends up in its lang root in multihost mode

Fixes #4105

hugolib/site_render.go

@@ -16,6 +16,7 @@ package hugolib
 import (
 	"fmt"
 	"path"
+	"strings"
 	"sync"
 
 	"github.com/gohugoio/hugo/helpers"
@@ -380,6 +381,13 @@ func (s *Site) renderAliases() error {
 					a = path.Join(a, f.Path)
 				}
 
+				lang := p.Lang()
+
+				if s.owner.multihost && !strings.HasPrefix(a, "/"+lang) {
+					// These need to be in its language root.
+					a = path.Join(lang, a)
+				}
+
 				if err := s.writeDestAlias(a, plink, p); err != nil {
 					return err
 				}