Ok, this comes close, as long as i do not use comma's as separator:
$modified = preg_replace('#\d+([.,]\d+)?#e', 'number_format("$0" * 0.8,2, ",", ".")', $text);
This prints out:
This sentence has not 4,40 no 0,80, 1,60, 2,40 or 3,60 words in it, not 8,00 either
for this sentence:
$text = "This sentence has not 5.50 no 1, 2, 3 or 4.50 words in it, not 10 either";
Which is almost perfect for us Dutchies. I always want things perfect, so the only thing that bugs me is the round number 8. I'd like that to be just 8, not 8,00 ;-)