htaccess redirect except homepage and language versions of the site

I have a WordPress site www.myoldsite.com

The homepage of the site has nothing but 4 links.

English - links to my new site www.mynewsite.com
French - links to www.myoldsite.com/fr
German - links to www.myoldsite.com/de
Russian - links to www.myoldsite.com/ru

I want all old site pages to be redirected to the new site’s home page except the homepage and language versions of the site.

www.myoldsite.com should not be redirected 
www.myoldsite.com/fr should not be redirected 
www.myoldsite.com/de should not be redirected 
www.myoldsite.com/de should not be redirected 

but anything else after forward slash should be redirected to the new site’s home page. i.e.

www.myoldsite.com/page1
www.myoldsite.com/page2

etc. should all be redirected to www.mynewsite.com.

Can anyone please help?

Below is my htaccess file

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress