How to add an extra part to the url using .htaccess?

Question

htaccess

Hello!
There is a site domain.com
When you enter the site, the script for determining the country is triggered and adds two letters of the country to the url.
Let's say we came from Russia and then we get to domain.com/ru/
How to make the server perceive this address as domain.com/?lg=ru?
Of course, any files/directories should be processed normally
. For example:

domain.com/ru/account/ -> domain.com/account/?lg=ru
domain.com/ru/php/1/order.php -> domain.com/php/1/order.php?lg=ru

And another point
The site should not crash when adding or not adding a slash at the end
. For example:

domain.com/ru/account/ -> domain.com/account/?lg=ru
domain.com/ru/account -> domain.com/account/?lg=ru

How to implement this? I try it myself, it doesn't work :(
Who fumbles in htaccess, please help!

Answer 1

Here's a starting example for you:

RewriteEngine On
RewriteRule ^([^/]+)(/.+)$ $2?lg=$1 [QSA,D]
RewriteRule ^(.+)$ index.php?p=$1 [QSA]

in index.php

<?php
print_r($_REQUEST);

and run with this request:
localhost/ru/test/fed?f=f
you get the following response:
further modify to taste

Answer 2

RewriteCond %{REQUEST_URI} ^(.*)/dir1/(.*)$ 
# если строка содержит /dir1/ 
RewriteRule ^(.*)$ %1/dir2/%1 [R=301,L]