Upload excel sheet to google translate (and pull the result) via VBA

So in another thread, I figured out how to load XLSB/XLSX files into a variable and put them into the body of a POST request.
However, it seems like I haven't figured it all out - as google translate doesn't give me the translation.
I'd love some feedback on what I am missing here.

Code I have so far, which saves a workbook and loads it into a variable - and then tries to upload it to Google Translate:

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Either the responsetext is empty, has "The translation of file " and then nothing or "Sorry, the page you requested contains a file type (application/octet-stream) we are unable to translate".
All with the same code.

Ahhh, the fun of VBA - I would really love some help here.
(Hoping that Kyle123 is still around... ;-))