User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0 Waterfox/56.3

Steps to reproduce:

1. Install Firefox in a different language than host system (e.g. English Firefox 73 on German Windows 10)
2. Set "intl.regional_prefs.use_os_locales" to "true", restart Firefox
3. Open certificate viewer on any HTTPS-enabled domain like https://example.com
4. Observe fields with dates in the certificate viewer (Not Before, Not After, SCT Timestamp)

about:support should say something like this:

Internationalization & Localization
Application Settings
Requested Locales ["en-US"]
Available Locales ["en-US"]
App Locales ["en-US"]
Regional Preferences ["de-DE"] <-- should match host OS
Default Locale "en-US"

Actual results:

Certificate viewer shows dates in the Firefox-installed locale ("en-US", "11/28/2018, 1:00:00 AM")

Expected results:

Certificate viewer should show dates in host OS locale ("de-DE", "28.11.2018, 01:00:00") like it does in other places like History/Bookmarks/Library. Or use the same unambiguous date format like in Add-ons and about:protections ("November 28, 2018").