@pico As you can see on https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toLocaleString, the parameters supplied are only used if the Intl.DateTimeFormat API is supported. We don't support that API and hence you can't get any time zone support, unfortunately.
I would suggest you to manually extract the components using getUTCHours, getUTCMinutes, getUTCSeconds etc. and then apply your time zone offset and DST formula manually, which should be doable if you know you will only be using a fixed time zone.