Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/locale/catalog.go

@@ -0,0 +1,54 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package locale // import "miniflux.app/v2/internal/locale"
+
+import (
+	"embed"
+	"encoding/json"
+	"fmt"
+)
+
+type translationDict map[string]interface{}
+type catalog map[string]translationDict
+
+var defaultCatalog catalog
+
+//go:embed translations/*.json
+var translationFiles embed.FS
+
+// LoadCatalogMessages loads and parses all translations encoded in JSON.
+func LoadCatalogMessages() error {
+	var err error
+	defaultCatalog = make(catalog)
+
+	for language := range AvailableLanguages() {
+		defaultCatalog[language], err = loadTranslationFile(language)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func loadTranslationFile(language string) (translationDict, error) {
+	translationFileData, err := translationFiles.ReadFile(fmt.Sprintf("translations/%s.json", language))
+	if err != nil {
+		return nil, err
+	}
+
+	translationMessages, err := parseTranslationMessages(translationFileData)
+	if err != nil {
+		return nil, err
+	}
+
+	return translationMessages, nil
+}
+
+func parseTranslationMessages(data []byte) (translationDict, error) {
+	var translationMessages translationDict
+	if err := json.Unmarshal(data, &translationMessages); err != nil {
+		return nil, fmt.Errorf(`invalid translation file: %w`, err)
+	}
+	return translationMessages, nil
+}