I understand the issue you're having and will try my best to assist you in this matter.
The issue you're having with the German characters most of the time happens due to not setting up the database collation correctly. From the WPML debug info, I can see that the Database collation is currently set to "utf8mb4_unicode_520_ci".
Would you please check by changing the collation to "utf8mb4_unicode_ci" and defining the charset to "utf8mb4"?
[ utf8mb4_unicode_ci is based on the official Unicode rules for universal sorting and comparison, which sorts accurately in a wide range of languages. ]
1) Open the wp-config.php file of your site root folder for editing and make sure that the following code is defined.
define( 'DB_CHARSET', 'utf8mb4' );
2) Please log in to the database using a tool like PHPMyAdmin and make sure that the collation for the database(including the Tables) is set to "utf8mb4_unicode_ci". You can reach out to the hosting support to verify if the Collation is configured correctly.
After updating the collation and charset, please retranslate the page and check if the umlauts characters are showing properly or not.
Let me know the update. I will be happy to help if you need further assistance in this matter.
