Translate Form Plugin validation

I need to translate the form plugin validation messages. How can this be done? Right now they look like this:
Validation failed: Invalid input in "Email"

I figured out how to change to second part by doing the following:

validate:
     required: true
     message: 'Bitte überprüfen Sie Ihre Eingaben' 

But I have no Idea how to translate the first part saying Validation failed:
Can somebody help me out on this?

Check translation overrides, the string is contained in system/languages/en.yaml

Had to add german (de) as only supported language to system.yaml to get the output in german. thanks.
but another question: if I'd like to change that text…
I had a look in the form template and found the following {{ "FORM.VALIDATION_FAIL"|t|raw }}. To translate/change this I tried

FORM:
  VALIDATION_FAIL: '<b>Test:</b>' 

inside my languages de.yaml but it didn't work. Whats wrong with it? Any clue?

Ah nevermind. I'm really sorry for the inconvenience. I edited the wrong file >.<
Works like a charm! Thanks!