Normally, TimeStory tracks your Mac’s system settings to determine your preferred language and date formats, and uses that to present things like month names, weekday names, and formatted dates.
But sometimes you may be preparing a document for others who use a different language or preferred format than you. In such cases, TimeStory lets you override your Mac’s settings for a specific document, so you all see the same thing. (This is pretty rare; usually, you want everyone to see a document according to their own preferences!)
Choose, and TimeStory will present a dialog which lets you switch between using the Mac’s system settings or using a specific language and region (e.g., US-English).
Note that this only affects the display of dates within the timeline. Fields in the Inspector, in Quick Entry, in Go To Date, and so on, will still always use your Mac’s preferred settings, since those fields are meant for date entry and won’t be visible in any exported timeline.