TYPO3 10.4.x site configuration language parameter - migrate from realurl

Question

In one of my projects I update TYPO3 from 6.2 to 10.4.
In the older version I used this grown historical realurl config:

$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['realurl'] = array (
    '_DEFAULT' => array (
        'preVars' => array(
            array(
                'GETvar' => 'L',
                'valueMap' => array(
                    //'de' => '0',
                    //'en' => '1'
                ),
                'valueDefault' => 'de',
                //'noMatch' => 'bypass',
            ),
        )
);

With this config I get these url's:

for start page: www.mydomain.de/

for sub pages: www.mydomain.de/de/subpage.html

With the new site configuration I use only the default entry for the language. But now I get no more "/de/" in the url. I know when I use the siteLanguage -> base field with "/de/" instead of "/", I get the "/de/" in the URL.

The stupid thing is that the "/de/" is also attached to the start page where it shouldn't go.

Can anyone give me a hint?

current site config:

base: 'https://www.domain.de'
baseVariants:
  -
    base: 'http://2021.domain.de/'
    condition: 'applicationContext == "Development"'
languages:
  -
    title: Deutsch
    enabled: true
    base: /
    typo3Language: de
    locale: de_DE.UTF-8
    iso-639-1: de
    websiteTitle: ''
    navigationTitle: Deutsch
    hreflang: de-de
    direction: ltr
    flag: de
    languageId: 0
rootPageId: 1
routeEnhancers:
  PageTypeSuffix:
    type: PageType
    default: .html
    index: ''
    map:
      .html: 0
      sitemap.xml: 1533906435