With a browser, how do I know which decimal separator does the operating system use?

Here is a simple JavaScript function that will return this information. Tested in Firefox, IE6, and IE7. I had to close and restart my browser in between every change to the setting under Control Panel / Regional and Language Options / Regional Options / Customize. However, it picked up not only the comma and period, but also oddball custom things, like the letter "a".

function whatDecimalSeparator() {
    var n = 1.1;
    n = n.toLocaleString().substring(1, 2);
    return n;
}

function whatDecimalSeparator() {
    var n = 1.1;
    n = n.toLocaleString().substring(1, 2);
    return n;
}

console.log('You use "' + whatDecimalSeparator() + '" as Decimal seprator');

Does this help?

Retrieving separators for the current or a given locale is possible using Intl.NumberFormat#formatToParts.

function getDecimalSeparator(locale) {
    const numberWithDecimalSeparator = 1.1;
    return Intl.NumberFormat(locale)
        .formatToParts(numberWithDecimalSeparator)
        .find(part => part.type === 'decimal')
        .value;
}

It only works for browsers supporting the Intl API. Otherwise it requires an Intl polyfill

Examples:

> getDecimalSeparator()
"."
> getDecimalSeparator('fr-FR')
","

Bonus:

We could extend it to retrieve either the decimal or group separator of a given locale:

function getSeparator(locale, separatorType) {
        const numberWithGroupAndDecimalSeparator = 1000.1;
        return Intl.NumberFormat(locale)
            .formatToParts(numberWithGroupAndDecimalSeparator)
            .find(part => part.type === separatorType)
            .value;
    }

Examples:

> getSeparator('en-US', 'decimal')
"."
> getSeparator('en-US', 'group')
","
> getSeparator('fr-FR', 'decimal')
","
> getSeparator('fr-FR', 'group')
" "

Ask the user, do not guess. Have a setting for it in your web application.

Edited to add:

I think it is ok to guess the default setting that works ok, say, 95% of the time. What I meant was that the user should still be able to override whatever guesses the software made. I've been frustrated too many times already when a software tries to be too smart and does not allow to be corrected.