how do I attach content of .COM..〈=ru to .RU website

I have once license for my .COM website which is in Enlish language.

The website is also translated in Russian and is outputed via .COM...&lang=ru.

I have bought a Russian extention .RU for the same domain.

I would like my .RU domain to display the content of .COM...&lang=ru while maintaing its .RU extention.

So when Russian language is selected on .COM, I need .COM...&lang=ru to be redirected to .RU 

How do I get this done, please?

It's not possible in the vanilla script, nor will you likely find an explanation in the forums. You can have http://www.example.ru/ redirect to http://www.example.com/index.php?lang=ru, but the site won't use the other domain.

Assuming you've access to .htaccess, anything is possible. The below code will redirect any user whose browser language is set to Russian:

RewriteCond %{HTTP_HOST} ([^.]+)\.yoursite.com [NC]
RewriteCond %{HTTP:Accept-Language} (ru) [NC]
RewriteRule ^$ http://www.yoursite.ru [L,R=301]

RewriteCond %{HTTP_HOST} ([^.]+)\.yoursite.ru [NC]
RewriteCond %{HTTP:Accept-Language} (ru) [NC]
RewriteRule ^$ /index.php?lang=ru [L,R=301] 

I should add that Dolphin selects the appropriate language based on the language settings for the user's browser. That's also why the default language option doesn't always work.