You may get a better response if you say what model year 644 you have, and whether it is a B644, C644 (or even a GT644 in some years). Hopefully, the following will work for you.
If you can track down someone with the correct manual in English, ask them for the reference number of the book. It normally appears in the footer of each page. There will be a sequence of date: month/year; language, then a long number, then some more letters and numbers. It is the long number in the middle that you want.
Then put the URL string below into your browser's address bar, BUT substitute the number '1269361' in the string I have given, with the one you have got from the correct manual. That should take you to the Hymer document download site and give you a pdf of the correct manual.