I believe your issue was that you set your bot's language to "de" (German).
Then you enabled translation in your app, so the translator was translating from the language set in the app, to the bot's language (German). Since your bot did not have any responses for German phrases, it replied with its default response.
This translator will only occur when you set a translation language in an app or web client.
It will translate to the language set in the bot's "Voice" page settings.
Since your bot is English, you should set its "language" in its "Voice" page settings to "en" or "en-US".
This is the default, so you must have changed it to German at some point.
If you review your bot's conversation logs, you will see German instead of English.