Clean up template locale usage (#27856) (#27857)

Backport #27856

The only conflict is `ThemeName` in `500.tmpl`, it has been resolved
manually by keeping using old
`{{.SignedUser.Theme}}{{else}}{{DefaultTheme}}`

services/mailer/mail_repo.go

@@ -12,7 +12,6 @@ import (
 	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/setting"
-	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/translation"
 )
 
@@ -65,6 +64,7 @@ func sendRepoTransferNotifyMailPerLang(lang string, newOwner, doer *user_model.U
 	}
 
 	data := map[string]any{
+		"locale":      locale,
 		"Doer":        doer,
 		"User":        repo.Owner,
 		"Repo":        repo.FullName(),
@@ -72,10 +72,6 @@ func sendRepoTransferNotifyMailPerLang(lang string, newOwner, doer *user_model.U
 		"Subject":     subject,
 		"Language":    locale.Language(),
 		"Destination": destination,
-		// helper
-		"locale":    locale,
-		"Str2html":  templates.Str2html,
-		"DotEscape": templates.DotEscape,
 	}
 
 	if err := bodyTemplates.ExecuteTemplate(&content, string(mailRepoTransferNotify), data); err != nil {