Add roles and versions as new dimensions (in addition to language)

See the main issue #13776 for details.

Fixes #519
Fixes #13680
Fixes #13663
Fixes #13776
Fixes #13855
Fixes #13648
Fixes #13996
Fixes #14001
Fixes #14031
Fixes #13818
Fixes #13196

modules/config_integration_test.go

@@ -0,0 +1,69 @@
+// Copyright 2025 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 modules_test
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/hugolib"
+)
+
+func TestMountsProjectDefaults(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+[languages]
+[languages.en]
+weight = 1
+[languages.sv]
+weight = 2
+
+[module]
+[[module.mounts]]
+source = 'content'
+target = 'content'
+lang = 'en'
+-- content/p1.md --
+
+`
+	b := hugolib.Test(t, files)
+
+	b.Assert(len(b.H.Configs.Modules), qt.Equals, 1)
+	projectMod := b.H.Configs.Modules[0]
+	b.Assert(projectMod.Path(), qt.Equals, "project")
+	mounts := projectMod.Mounts()
+	b.Assert(len(mounts), qt.Equals, 7)
+	contentMount := mounts[0]
+	b.Assert(contentMount.Source, qt.Equals, "content")
+	b.Assert(contentMount.Sites.Matrix.Languages, qt.DeepEquals, []string{"en"})
+}
+
+func TestMountsLangIsDeprecated(t *testing.T) {
+	t.Parallel()
+	files := `
+-- hugo.toml --
+[module]
+[[module.mounts]]
+source = 'content'
+target = 'content'
+lang = 'en'
+-- layouts/all.html --
+All.
+`
+
+	b := hugolib.Test(t, files, hugolib.TestOptInfo())
+	b.AssertLogContains("deprecated")
+}