Move middlewares to web/middleware (#14480)

Co-authored-by: 6543 <6543@obermui.de>

modules/web/middleware/locale.go

@@ -0,0 +1,49 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package middleware
+
+import (
+	"net/http"
+
+	"code.gitea.io/gitea/modules/translation"
+
+	"github.com/unknwon/i18n"
+	"golang.org/x/text/language"
+)
+
+// Locale handle locale
+func Locale(resp http.ResponseWriter, req *http.Request) translation.Locale {
+	// 1. Check URL arguments.
+	lang := req.URL.Query().Get("lang")
+	var changeLang = lang != ""
+
+	// 2. Get language information from cookies.
+	if len(lang) == 0 {
+		ck, _ := req.Cookie("lang")
+		if ck != nil {
+			lang = ck.Value
+		}
+	}
+
+	// Check again in case someone modify by purpose.
+	if lang != "" && !i18n.IsExist(lang) {
+		lang = ""
+		changeLang = false
+	}
+
+	// 3. Get language information from 'Accept-Language'.
+	// The first element in the list is chosen to be the default language automatically.
+	if len(lang) == 0 {
+		tags, _, _ := language.ParseAcceptLanguage(req.Header.Get("Accept-Language"))
+		tag, _, _ := translation.Match(tags...)
+		lang = tag.String()
+	}
+
+	if changeLang {
+		SetCookie(resp, "lang", lang, 1<<31-1)
+	}
+
+	return translation.NewLocale(lang)
+}