Move internal packages to an internal folder

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

internal/locale/locale.go

@@ -0,0 +1,28 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package locale // import "miniflux.app/v2/internal/locale"
+
+// AvailableLanguages returns the list of available languages.
+func AvailableLanguages() map[string]string {
+	return map[string]string{
+		"en_US": "English",
+		"es_ES": "Español",
+		"fr_FR": "Français",
+		"de_DE": "Deutsch",
+		"pl_PL": "Polski",
+		"pt_BR": "Português Brasileiro",
+		"zh_CN": "简体中文",
+		"zh_TW": "繁體中文",
+		"nl_NL": "Nederlands",
+		"ru_RU": "Русский",
+		"it_IT": "Italiano",
+		"ja_JP": "日本語",
+		"tr_TR": "Türkçe",
+		"el_EL": "Ελληνικά",
+		"fi_FI": "Suomi",
+		"hi_IN": "हिन्दी",
+		"uk_UA": "Українська",
+		"id_ID": "Bahasa Indonesia",
+	}
+}