On Windows if Picard is set to use the system language, it uses the locale.getdefaultlocale(), which on Windows returns the region settings. This is different from the UI settings, e.g. I can have my UI language set to English (because I prefer this or I am native English speaker), but my region settings to Germany (because I live in Germany and want to use e.g. local currency and date formatting).
And this Stackoverflow answer: