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}}`

modules/context/context.go

@@ -157,7 +157,6 @@ func Contexter() func(next http.Handler) http.Handler {
 			ctx.Data["Context"] = ctx // TODO: use "ctx" in template and remove this
 			ctx.Data["CurrentURL"] = setting.AppSubURL + req.URL.RequestURI()
 			ctx.Data["Link"] = ctx.Link
-			ctx.Data["locale"] = ctx.Locale
 
 			// PageData is passed by reference, and it will be rendered to `window.config.pageData` in `head.tmpl` for JavaScript modules
 			ctx.PageData = map[string]any{}