Yes, I have the "24-hour time" checked under "General → Date & Time". The language/region is "United States", and the languages in use are: "English (US) — Primary", and "Suomi" (Finnish).
The language/region settings are a mix otherwise:
Temp: °C
Measurement system: US
First day of week: Monday
Date format: 2025-08-19
.. because I'm from Finland but live in the U.S. 🙂
I switched from Sonoma to Sequoia exactly when I switched computers and used WorkingHours last on the old system. Apple tied some of the time settings more closely with Region in Sequoia, so that might be the reason why this works differently now. If there's a problem making the app follow the system time setting reliably, a 12/24h option within the app's settings would work just as well. Optimally, the date format would also be adjustable, although the US format date is less distracting than the 12-hour time.