Edit: Oh forgot for a moment that we were in the API Feedback thread, I tried to help you to find a solution when you only wanted to share the feedback. 
Yes that’s what I understood, thank you for clarifing that I was a bit unclar in my response:
So Whisper thought the content you provided was in welsh (as a explained in the previous post) and then Whisper tried to transcribe the content in Welsh.
This is like using Google Assistant but setting it to English and then speaking in german. You’ll get some text out of it but the text might not make sense. This matches what you described when you were putting the text in Google translator.
So there was no translation happening, Whisper only thought that you provided welsh content due to the first 30 seconds of the file.