cherry-pick: all: fix some races

Updates #3087.

Squashed commit of the following:

commit e888ce524f286f3c34e14e0086336b65a95fa020
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon May 17 16:34:10 2021 +0300

    all: fix some races

internal/home/i18n.go

@@ -86,7 +86,13 @@ func handleI18nChangeLanguage(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	config.Language = language
+	func() {
+		config.Lock()
+		defer config.Unlock()
+
+		config.Language = language
+	}()
+
 	onConfigModified()
 	returnOK(w)
 }