attempt fallback to "other" plural form (#174)

Author: nicksnyder

v2/i18n/localizer.go

@@ -121,6 +121,7 @@ func (l *Localizer) LocalizeMessage(msg *Message) (string, error) {
 // }
 
 // LocalizeWithTag returns a localized message and the language tag.
+// It may return a best effort localized message even if an error happens.
 func (l *Localizer) LocalizeWithTag(lc *LocalizeConfig) (string, language.Tag, error) {
 	messageID := lc.MessageID
 	if lc.DefaultMessage != nil {
@@ -144,16 +145,26 @@ func (l *Localizer) LocalizeWithTag(lc *LocalizeConfig) (string, language.Tag, e
 			}
 		}
 	}
+
 	tag, template := l.getTemplate(messageID, lc.DefaultMessage)
 	if template == nil {
 		return "", language.Und, &MessageNotFoundErr{messageID: messageID}
 	}
+
 	pluralForm := l.pluralForm(tag, operands)
 	if pluralForm == plural.Invalid {
 		return "", language.Und, &pluralizeErr{messageID: messageID, tag: tag}
 	}
+
 	msg, err := template.Execute(pluralForm, templateData, lc.Funcs)
 	if err != nil {
+		// Attempt to fallback to "Other" pluralization in case translations are incomplete.
+		if pluralForm != plural.Other {
+			msg2, err2 := template.Execute(plural.Other, templateData, lc.Funcs)
+			if err2 == nil {
+				return msg2, tag, err
+			}
+		}
 		return "", language.Und, err
 	}
 	return msg, tag, nil

v2/i18n/localizer_test.go

@@ -4,6 +4,7 @@ import (
 	"reflect"
 	"testing"
 
+	"github.com/nicksnyder/go-i18n/v2/internal/plural"
 	"golang.org/x/text/language"
 )
 
@@ -266,6 +267,34 @@ func TestLocalizer_Localize(t *testing.T) {
 			},
 			expectedLocalized: "I have 1 cat",
 		},
+		{
+			name:            "plural count missing one, default message",
+			defaultLanguage: language.English,
+			acceptLangs:     []string{"en"},
+			conf: &LocalizeConfig{
+				PluralCount: 1,
+				DefaultMessage: &Message{
+					ID:    "Cats",
+					Other: "I have {{.PluralCount}} cats",
+				},
+			},
+			expectedLocalized: "I have 1 cats",
+			expectedErr:       pluralFormNotFoundError{messageID: "Cats", pluralForm: plural.One},
+		},
+		{
+			name:            "plural count missing other, default message",
+			defaultLanguage: language.English,
+			acceptLangs:     []string{"en"},
+			conf: &LocalizeConfig{
+				PluralCount: 2,
+				DefaultMessage: &Message{
+					ID:  "Cats",
+					One: "I have {{.PluralCount}} cat",
+				},
+			},
+			expectedLocalized: "",
+			expectedErr:       pluralFormNotFoundError{messageID: "Cats", pluralForm: plural.Other},
+		},
 		{
 			name:            "plural count other, default message",
 			defaultLanguage: language.English,