I believe there are some bugs with non-American locale settings while attempting to download the data about your account.

My setup: Windows 10. Affects both Edge and Firefox.

Steps to reproduce:
1. Set the regional date setting in your OS to Poland, for example.
2. On Twitch, go to Settings > Security > Download your data.
3. Select custom range from June 1st, 2025 to July 31st, 2025.
Clicking "Update" causes time format errors to appear in the JavaScript console, and the "Request" button does not become active.

Changing your Twitch display language to English (US) seems to mess with the expected manual input, changing the letters appearing when you delete the date from DD.MM.YYYY to MM/DD/YYYY. Neither of them let me properly select the dates, though, while my OS regional setting was set to Poland.