I actually found a workaround.
Systemlanguage is stored in the code itself not in the catabase.
If you want to change the Systemlanguage do the following:
- Go to the Table Language in your DB
- Copy the id of your Systemlanguage
- Make sure to remove the 0x prefix of the ID
- Search your Code for the ID you’ve copied before
- You should be able to find the ID in the following Files: common.js, Defaults.php, shopware.js and common.js
- I replaced the ID with the one i wanted as default Language in the above Files
Be aware that might not be safe to do this option and make a backup before.
For me this Solution worked, i just needed to set all Names of Product and so on in the Default Language