i18n: Add Lower() function and add code comments

Signed-off-by: Michael Mayer <michael@photoprism.app>

internal/i18n/i18n.go

@@ -50,10 +50,17 @@ func msgParams(msg string, params ...interface{}) string {
 	return msg
 }
 
+// Msg returns a translated message string.
 func Msg(id Message, params ...interface{}) string {
 	return msgParams(gotext.Get(Messages[id]), params...)
 }
 
+// Error returns a translated error message.
 func Error(id Message, params ...interface{}) error {
 	return errors.New(Msg(id, params...))
 }
+
+// Lower returns the untranslated message as a lowercase string for use in logs.
+func Lower(id Message, params ...interface{}) string {
+	return strings.ToLower(msgParams(Messages[id], params...))
+}

internal/i18n/i18n_test.go

@@ -75,3 +75,21 @@ func TestError(t *testing.T) {
 		assert.EqualError(t, errDefault, "A cat already exists")
 	})
 }
+
+func TestLower(t *testing.T) {
+	t.Run("ErrAlreadyExists", func(t *testing.T) {
+		msg := Lower(ErrAlreadyExists, "Eine Katze")
+		assert.Equal(t, "eine katze already exists", msg)
+		SetLocale("de")
+		errGerman := Lower(ErrAlreadyExists, "Eine Katze")
+		assert.Equal(t, errGerman, "eine katze already exists")
+		SetLocale("")
+		errDefault := Lower(ErrAlreadyExists, "Eine Katze")
+		assert.Equal(t, errDefault, "eine katze already exists")
+	})
+
+	t.Run("ErrForbidden", func(t *testing.T) {
+		msg := Lower(ErrForbidden, "A cat")
+		assert.Equal(t, "permission denied", msg)
+	})
+}