Can someone help me to translate this automator workflow to applescript?
You can use a "Run Shell Script" action and sips to downscale your pictures to any value. Add a "Run Shell Script" action, make sure the input method is set to "stdin" (should be the default) and insert the following script

SCALEFACTOR=500
while IFS= read file; do
    sips --resampleWidth $(($(sips -g pixelWidth "$file" |
                                  sed -n -e '/pixelWidth/s/.*pixelWidth: //p')
                            * $SCALEFACTOR / 1000)) "$file" --out "${file%/*}/resized-${file##*/}"
done


Shell arithmetic only supports integer values, so you'll have to specify the scale factor in thousands (e.g. 424 for 42.4%)