langs/i18n: Add workaround for known language, but missing plural rule error

Closes #7798

langs/i18n/i18n_test.go

@@ -199,6 +199,19 @@ other = "{{ .Count }} minuttar lesing"`),
 		expected:     "3 minuttar lesing",
 		expectedFlag: "3 minuttar lesing",
 	},
+	// https://github.com/gohugoio/hugo/issues/7798
+	{
+		name: "known-language-missing-plural",
+		data: map[string][]byte{
+			"oc.toml": []byte(`[oc]
+one =  "abc"`),
+		},
+		args:         1,
+		lang:         "oc",
+		id:           "oc",
+		expected:     "abc",
+		expectedFlag: "abc",
+	},
 	// https://github.com/gohugoio/hugo/issues/7794
 	{
 		name: "dotted-bare-key",
@@ -292,7 +305,7 @@ func TestI18nTranslate(t *testing.T) {
 				} else {
 					expected = test.expected
 				}
-				actual = doTestI18nTranslate(t, test, v)
+				actual = doTestI18nTranslate(c, test, v)
 				c.Assert(actual, qt.Equals, expected)
 			})
 		}

langs/i18n/translationProvider.go

@@ -15,6 +15,7 @@ package i18n
 
 import (
 	"encoding/json"
+	"strings"
 
 	"github.com/gohugoio/hugo/common/herrors"
 	"golang.org/x/text/language"
@@ -95,6 +96,14 @@ func addTranslationFile(bundle *i18n.Bundle, r source.File) error {
 
 	_, err = bundle.ParseMessageFileBytes(b, name)
 	if err != nil {
+		if strings.Contains(err.Error(), "no plural rule") {
+			// https://github.com/gohugoio/hugo/issues/7798
+			name = artificialLangTagPrefix + name
+			_, err = bundle.ParseMessageFileBytes(b, name)
+			if err == nil {
+				return nil
+			}
+		}
 		return errWithFileContext(_errors.Wrapf(err, "failed to load translations"), r)
 	}