To reproduce visit a French wiki, open the Javascript console and execute
mw.loader.load('mediawiki.language.numbers');Wait until it is loaded, then execute
mw.language.commafy(1234.567, '#,##0.##');
Expected result (according to http://www.unicode.org/reports/tr35/tr35-numbers.html#Number_Format_Patterns)
"1 234,57"
Actual result:
"1 234,56"