langs/i18n: Improve reserved key error message

Closes #14061

Author: jmooring

langs/i18n/i18n_integration_test.go

@@ -16,6 +16,7 @@ package i18n_test
 import (
 	"testing"
 
+	qt "github.com/frankban/quicktest"
 	"github.com/gohugoio/hugo/hugolib"
 )
 
@@ -144,3 +145,23 @@ description: {{ T "description" }}|
 
 	b.AssertFileContent("public/index.html", `description: This is a description from i18n.|`)
 }
+
+// See issue #14061
+func TestI18nReservedKeyScalar(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['home','page','rss','section','sitemap','taxonomy','term']
+-- i18n/en.toml --
+a = 'a translated'
+description = 'description translated'
+b = 'b translated'
+
+`
+
+	b, err := hugolib.TestE(t, files)
+
+	b.Assert(err, qt.IsNotNil)
+	b.Assert(err.Error(), qt.Contains, "failed to load translations: reserved keys [description] mixed with unreserved keys [a b]: see the lang.Translate documentation for a list of reserved keys")
+}

langs/i18n/translationProvider.go

@@ -114,7 +114,11 @@ func addTranslationFile(bundle *i18n.Bundle, r *source.File) error {
 				return nil
 			}
 		}
-		return errWithFileContext(fmt.Errorf("failed to load translations: %w", err), r)
+		var guidance string
+		if strings.Contains(err.Error(), "mixed with unreserved keys") {
+			guidance = ": see the lang.Translate documentation for a list of reserved keys"
+		}
+		return errWithFileContext(fmt.Errorf("failed to load translations: %w%s", err, guidance), r)
 	}
 
 	return nil