Language is a personal setting. Open Profile and choose your Language, or use Language in the sidebar user menu.
Time format applies to the whole business. Open Settings → Business and set Time format to either 24-hour or 12-hour.
Save the changes and refresh the affected page. The time format changes how times are displayed; it does not change the location's time zone or working hours.
Customers choose their own language on public pages, so it doesn't need to match the language your team uses.
After changing regional settings, check a booking calendar, a queue time, and a public page to make sure the result is clear.