Here's a proper multisite solution:

**Install WordPress** directly on the root of your domain `mysite.de`. After that, setup a multisite and chose the `sub domain` option (Yes, subdomain. It's far more flexible but you can use it with subdirectories as well).

WordPress will ask you to update `wp-config.php` and `.htaccess`. To the `wp-config.php` add the following constants (in addition to the one provided by WP)

    define( "COOKIEDOMAIN", "" );
    define( "ADMIN_COOKIE_PAHT", "/" );
    define( 'SITECOOKIEPATH', '/' );

Don't use the .htaccess code provided by WordPress but use **instead** this one:

    RewriteEngine On
    RewriteBase /
    RewriteRule ^index\.php$ - [L]
    
    # add a trailing slash to /wp-admin
    RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]
    
    RewriteCond %{REQUEST_FILENAME} -f [OR]
    RewriteCond %{REQUEST_FILENAME} -d
    RewriteRule ^ - [L]
    RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
    RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
    RewriteRule . index.php [L]

**Go to the network admin menu** and create two sites, one for each language. You will get asked for the site address (subdomain). Just enter `de` for the German site and `en` for the English one. After the site has been created, edit it directly. _Now_ you can edit the complete URL. Just set it to `http://mysite.de/de` and `http://mysite.de/en`. Do this also for the French site.

Now you have four sites in your network, probably with the following IDs:


    1 mysite.de/
    2 mysite.de/de
    3 mysite.de/en
    4 mysite.de/fr


As you don't need the first one, you can leave it just empty. You just don't use it. Manage your content in the other thre sites. 

To redirect requests to `mysite.de/` add another line to your `.htaccess` above the others:

    RedirectMatch permanent "^/$" http://mysite.de/de

You now have a full functional network of two active sites for your content.

Finally a personal recommendation: There's a free plugin called [Multilingual Press](https://wordpress.org/plugins/multilingual-press/) that supports this concept of a multilingual setup in a network. Disclamer: I contributed to this plugin before and working for a company that develops this plugin.