I use the German language files, which are 100 % translated, but unfortunately we get this message for required fields:
Es gab ein Problem mit Ihrer Eingabe. Please review the fields below.
The first sentence ist translated and displayed correctly.
I have checked with loco translate, the second phrase is translated correctly, but for some reason the translation is not used.
I’m getting the full message translated when I switch my default WordPress installation to German language, please see this: Screenshot HZqSHBbq9e.png - Droplr
Loco Translate can override the default translation with a custom one. I would recommend you to disable Loco Translate and check if you have any Gravity Forms language file at /wp-content/languages/gravityforms/
If so, delete the file(s).
Also try going to /wp-content/languages/plugins/ and delete any file containing gravityforms as part of its name. Then go to your site Dashboard > Aktualisierungen and check for any pending updates for translations at the bottom of the page.
If none of the above helps, I would recommend you to open a support ticket for further assistance.