I need to format a Timestamp in a specific locale (not utc, not browser locale). But I must have the millisecond part of the date, too. My first attempt was second:'numeric' with the DateTimeFormat API:

new Intl.DateTimeFormat(
    'de-DE', // german as an example, user selectable
    { 
        year: 'numeric', month: 'numeric',  day: 'numeric', 
        hour: 'numeric', minute: 'numeric', 
        second: 'numeric',
        hour12: false
    }
)
.format(new Date()); // Date as an example

But the result is something like 26.11.2018, 09:31:04 and not 26.11.2018, 09:31:04,243.

Asked first at stackoverflow.