Move internal packages to an internal folder

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

internal/locale/locale_test.go

@@ -0,0 +1,23 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package locale // import "miniflux.app/v2/internal/locale"
+
+import "testing"
+
+func TestAvailableLanguages(t *testing.T) {
+	results := AvailableLanguages()
+	for k, v := range results {
+		if k == "" {
+			t.Errorf(`Empty language key detected`)
+		}
+
+		if v == "" {
+			t.Errorf(`Empty language value detected`)
+		}
+	}
+
+	if _, found := results["en_US"]; !found {
+		t.Errorf(`We must have at least the default language (en_US)`)
+	}
+}