Replace all usage of CopyOnWriteFs with OverlayFs

Fixes #9761

langs/i18n/integration_test.go

@@ -0,0 +1,57 @@
+// Copyright 2022 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 i18n_test
+
+import (
+	"testing"
+
+	"github.com/gohugoio/hugo/hugolib"
+)
+
+func TestI18nFromTheme(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- config.toml --
+[module]
+[[module.imports]]
+path = "mytheme"
+-- i18n/en.toml --
+[l1]
+other = 'l1main'
+[l2]
+other = 'l2main'
+-- themes/mytheme/i18n/en.toml --
+[l1]
+other = 'l1theme'
+[l2]
+other = 'l2theme'
+[l3]
+other = 'l3theme'
+-- layouts/index.html --
+l1: {{ i18n "l1"  }}|l2: {{ i18n "l2"  }}|l3: {{ i18n "l3"  }}
+
+`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+		},
+	).Build()
+
+	b.AssertFileContent("public/index.html", `
+l1: l1main|l2: l2main|l3: l3theme
+	`)
+}