I have Windows, and I have tried with the following languages:
- English (India) en_IN (This is my default)
- English (Germany) en_DE
Here are the settings from en_IN on Windows:

Here is how Portfolio Performance shows date and time with en_IN:


Here are the settings from en_DE on Windows:

Here is how Portfolio Performance shows date and time with en_DE:


So, here, for data display:
- Date is shown as DD-MMM-YYYY irrespective of the format, even though none of the system settings use this, and en_IN uses DD-MM-YYYY and en_DE uses DD/MM/YYYY. PP isn’t using the long date format either.
- Time is shown correctly with en_DE but not with en_IN even though the time format is the same in both cases.
For data input:
- Date format is irrespective of language DD-MM-YYYY and does not change when en_DE is selected.
- 12-hour clock is used with en_IN but 24-hour clock is used with en_DE even though both are set to 24-hours in the system settings.
Setting the language manually to English on Portfolio Performance changes the format to what I believe is English (US) format, with month before date:

Ironically, though, the data input screen does not change format:

This is not what I am looking for.