Is it possible to add response http headers in web.config?

The best way to do this would be the <customHeaders> element of the web.config file. Note that this only works for IIS version 7 and above.

The configuration to add your example header would be:

<configuration>
  <system.webServer>
    <httpProtocol>
      <customHeaders>
        <add name="Content-Language" value="*" />
      </customHeaders>
    </httpProtocol>
  </system.webServer>
</configuration>

For more information see the IIS “Custom Headers” Configuration Reference page