This is not "language". The whole idea to specify a language is outdated and generally bad. Think what happesn that your page if many pages is written with mixed fragment of Latin, your Perso-Arabic script, and, say, Devanāgarī and Cyrillic? Where your "defaults" will be applied? The whole idea is wrong. With Web, we use Unicode, which allows us to mix any number of different languages in one string.
All your pages need the following fragment:
<html>
<head>
<title>Some titlle</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<!--
</head>
<body>
<!--
</body>
</html>
Naturally, the text of ASP.NET or HTML file should also be saved in UTF-8.
What happens if you rely on the server/side defaults? Think about the interests of the users who may want to just save a page locally. Not having proper HTTP-EQUIV will screw up rendering.
It will work without RTL, but, for some second-order reasons, it's good to specify it in CSS. Please see:
direction — CSS | MDN[
^].
—SA