Can't set Global Var

Hi all,

I can’t get my head around a rather simple task, I’m afraid.
I’m creating a dual language website. For incompatibility reasons with a must have plugin (dsIDX), I can’t use WPML or qTranslate.
So I want to change wp_nav_menu myself. I have 2 menus (1MainEN = english and 1MainDE = german)

I have a simple form in a sidebar widget, where the user can chose the language:

<form method="POST" action="">
	<select name="IMmenuSelection" onchange='this.form.submit()'>
	  <option>English</option>
	  <option>Deutsch</option>
	</select>
</form>

In the child themes functions.php I defined a global var and get and set functions

global $IM_menu;
$IM_menu = "1MainEN";

// the function to get the global variable
function IM_menu_get_var()
        {
		global $IM_menu;
		return $IM_menu;
	}
// the function to set the global variable
function IM_menu_set_var($IMnew_value)
	{
		global $IM_menu;
		$IM_menu = $IMnew_value;
	}

in the child themes header.php I try to catch the users choice and set the global var $IM_menu accordingly.

<?php
	$IM_post_language = $_POST['IMmenuSelection']; //what did the user select
	if( $IM_post_language == "Deutsch") {
		IM_menu_set_var('1MainDE');
	}
	if( $IM_post_language == "English") {
		IM_menu_set_var('1MainEN');
	}
?>
<?php
	wp_nav_menu( array('container' => 'div', 'container_class' => 'menu-wrapper', 'container_id' => 'main-superfish-wrapper', 'menu_class'=> 'sf-menu',  'theme_location' => 'main_menu', 'menu' => IM_menu_get_var() ) );
?>

It reacts when the user selects a different language (DE) and shows the right menu (defaults to Home in the new menu), but once the user selects a different entry in the new menu (DE), it switches back to the English Menu – not recognizing the global var $IM_menu.

I’m out of ideas, as I try to get this straight for too long — tunnel view!
You can see the site here https://realtyswflorida.com

Any help is more than welcome!

[Moderator Note: No bumping, thank you.]