to parse English use this format: 'en_EN' . I had to guess. I have no idea where you would find these codes.
$fmt = numfmt_create( 'en_EN', NumberFormatter::DECIMAL );
$num = "1,234,567.891";
echo numfmt_parse($fmt, $num)."\n";
echo numfmt_parse($fmt, $num, NumberFormatter::TYPE_INT32)."\n";
If this doesn't work well all I can say is it shouldn't work because it makes no sense that you would repeat the code twice "en and EN" in one code. Stuff that makes no sense is hard to guess. I couldn't find a page that has these codes either. I guess documentation is bound to be incomplete since it's so voluminous. Documentation has a language and syntax unto itself yet there are no tutorials on understanding documentation.