Refactor locale number (#24134)

Before, the `GiteaLocaleNumber.js` was just written as a a drop-in
replacement for old `js-pretty-number`.

Actually, we can use Golang's `text` package to format.

This PR partially completes the TODOs in `GiteaLocaleNumber.js`:

> if we have complete backend locale support (eg: Golang "x/text"
package), we can drop this component.
> tooltip: only 2 usages of this, we can replace it with Golang's
"x/text/number" package in the future.

This PR also helps #24131

Screenshots:

<details>

![image](https://user-images.githubusercontent.com/2114189/232179420-b1b9974b-9d96-4408-b209-b80182c8b359.png)


![image](https://user-images.githubusercontent.com/2114189/232179416-14f36aa0-3f3e-4ac9-b366-7bd3a4464a11.png)

</details>

3 files changed, 79 insertions, 5 deletions

diff --git a/modules/translation/mock.go b/modules/translation/mock.go
new file mode 100644
index 0000000000..6ce66166aa
--- /dev/null
+++ b/modules/translation/mock.go
@@ -0,0 +1,27 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package translation
+
+import "fmt"
+
+// MockLocale provides a mocked locale without any translations
+type MockLocale struct{}
+
+var _ Locale = (*MockLocale)(nil)
+
+func (l MockLocale) Language() string {
+	return "en"
+}
+
+func (l MockLocale) Tr(s string, _ ...interface{}) string {
+	return s
+}
+
+func (l MockLocale) TrN(_cnt interface{}, key1, _keyN string, _args ...interface{}) string {
+	return key1
+}
+
+func (l MockLocale) PrettyNumber(v any) string {
+	return fmt.Sprint(v)
+}
diff --git a/modules/translation/translation.go b/modules/translation/translation.go
index 331da0f965..56cf1df2d4 100644
--- a/modules/translation/translation.go
+++ b/modules/translation/translation.go
@@ -15,17 +15,20 @@ import (
 	"code.gitea.io/gitea/modules/translation/i18n"
 
 	"golang.org/x/text/language"
+	"golang.org/x/text/message"
+	"golang.org/x/text/number"
 )
 
 type contextKey struct{}
 
-var ContextKey interface{} = &contextKey{}
+var ContextKey any = &contextKey{}
 
 // Locale represents an interface to translation
 type Locale interface {
 	Language() string
-	Tr(string, ...interface{}) string
-	TrN(cnt interface{}, key1, keyN string, args ...interface{}) string
+	Tr(string, ...any) string
+	TrN(cnt any, key1, keyN string, args ...any) string
+	PrettyNumber(v any) string
 }
 
 // LangType represents a lang type
@@ -135,6 +138,7 @@ func Match(tags ...language.Tag) language.Tag {
 type locale struct {
 	i18n.Locale
 	Lang, LangName string // these fields are used directly in templates: .i18n.Lang
+	msgPrinter     *message.Printer
 }
 
 // NewLocale return a locale
@@ -147,13 +151,24 @@ func NewLocale(lang string) Locale {
 	langName := "unknown"
 	if l, ok := allLangMap[lang]; ok {
 		langName = l.Name
+	} else if len(setting.Langs) > 0 {
+		lang = setting.Langs[0]
+		langName = setting.Names[0]
 	}
+
 	i18nLocale, _ := i18n.GetLocale(lang)
-	return &locale{
+	l := &locale{
 		Locale:   i18nLocale,
 		Lang:     lang,
 		LangName: langName,
 	}
+	if langTag, err := language.Parse(lang); err != nil {
+		log.Error("Failed to parse language tag from name %q: %v", l.Lang, err)
+		l.msgPrinter = message.NewPrinter(language.English)
+	} else {
+		l.msgPrinter = message.NewPrinter(langTag)
+	}
+	return l
 }
 
 func (l *locale) Language() string {
@@ -199,7 +214,7 @@ var trNLangRules = map[string]func(int64) int{
 }
 
 // TrN returns translated message for plural text translation
-func (l *locale) TrN(cnt interface{}, key1, keyN string, args ...interface{}) string {
+func (l *locale) TrN(cnt any, key1, keyN string, args ...any) string {
 	var c int64
 	if t, ok := cnt.(int); ok {
 		c = int64(t)
@@ -223,3 +238,8 @@ func (l *locale) TrN(cnt interface{}, key1, keyN string, args ...interface{}) st
 	}
 	return l.Tr(keyN, args...)
 }
+
+func (l *locale) PrettyNumber(v any) string {
+	// TODO: this mechanism is not good enough, the complete solution is to switch the translation system to ICU message format
+	return l.msgPrinter.Sprintf("%v", number.Decimal(v))
+}
diff --git a/modules/translation/translation_test.go b/modules/translation/translation_test.go
new file mode 100644
index 0000000000..83a40f1458
--- /dev/null
+++ b/modules/translation/translation_test.go
@@ -0,0 +1,27 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package translation
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/translation/i18n"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPrettyNumber(t *testing.T) {
+	// TODO: make this package friendly to testing
+
+	i18n.ResetDefaultLocales()
+
+	allLangMap = make(map[string]*LangType)
+	allLangMap["id-ID"] = &LangType{Lang: "id-ID", Name: "Bahasa Indonesia"}
+
+	l := NewLocale("id-ID")
+	assert.EqualValues(t, "1.000.000", l.PrettyNumber(1000000))
+
+	l = NewLocale("nosuch")
+	assert.EqualValues(t, "1,000,000", l.PrettyNumber(1000000))
+}