What I want to achieve?

I want to redirect to /de/ if the homepage (/) is called and the HTTP header Accept-Language begins with de of the visitor's browser but only for specific domains.

Here's my starting point:

As I have many domain specific redirect rules, I have multiple <If> directives. The following code snippet is reduced to a minimum.

<IfModule mod_rewrite.c>
 RewriteEngine on

 # Redirect serveral domain aliases
 RewriteCond %HTTP_HOST ^(www.)?mydomain.com$ [OR]
 RewriteCond %HTTP_HOST ^(www.)?my-domain.com$ [OR]
 RewriteCond %HTTP_HOST ^(www.)?anotherdomain.com$
 RewriteRule .* https://www.mywebsite.com/en/products/overview/ [R=301,L]

 RewriteCond %HTTP_HOST ^(karriere|jobs).areal-domain.at$
 RewriteRule .* https://www.mywebsite.com/de/ueber-uns/karriere/ [R=301,L]

 # Redirect language DE for homepage only of www.mywebsite.com
 # (the following rule is outside of the following otherwise it will not work actually
 RewriteCond %REQUEST_URI ^/?$
 RewriteCond %HTTP:Accept-Language ^de [NC]
 RewriteRule ^$ /de/ [R=301,QSA,L]

 <If "req('Host') =~ /(live.mywebsite.com|www.mywebsite.com|mywebsite.com|mywebsite.docker)/">
 RedirectMatch 301 ^/$ /en/
 RedirectMatch 301 ^/contact(/)?$ /en/about-us/contact/
 RedirectMatch 301 ^/customer-support(/)?$ /en/service-and-support/
 </If>

</IfModule>

This is my problem:

The apache v2.4.38 server does always send a HTTP status code 500 if I put the rewrite for the language redirection into the <If> directive as shown in the next snippet:

<IfModule mod_rewrite.c>
 RewriteEngine on

 # ...

 <If "req('Host') =~ /(live.mywebsite.com|www.mywebsite.com|mywebsite.com|mywebsite.docker)/">
 # Redirect language DE for homepage only of www.mywebsite.com
 RewriteCond %REQUEST_URI ^/?$
 RewriteCond %HTTP:Accept-Language ^de [NC]
 RewriteRule ^$ /de/ [R=301,QSA,L]

 RedirectMatch 301 ^/$ /en/
 RedirectMatch 301 ^/contact(/)?$ /en/about-us/contact/
 RedirectMatch 301 ^/customer-support(/)?$ /en/service-and-support/
 </If>

</IfModule>

What is the reason for this behaviour?

I read the documentation, but I couldn't find any information about my scenario.