If the user is not logged in to the WHMCS shop the default language for the Acronis module (custom fields by order) is English instead of German.
I've changed the default language in the file «/public_html/modules/servers/AcronisBackupService/app/Acronis/Standard/Helpers/LanguageMap.php»
-> const DEFAULT_LANGUAGE_ISO = 'de';
But it didn't solved my issue.
If I change the code in the file «iso-languages.json» (/public_html/modules/servers/AcronisBackupService/lang/iso-languages.json) like this
It doesn't change the langue on the user page. For this reason it seems as if the code will be oversteered by some other, higher ranked code.
Many thanks for your support.