I have only seen this bug in the config screen of the admin workspace but I guess this is general issue and is not specifically related to the config screen (but I could be wrong...)

How to reproduce:
Start your project without a config.xml file to load the default config. Time and date widgets will display value formatted in english (with AM/PM). But once the user select a value in the drop-down list of the combobox, the value will be formatted in the expected format.

Moreover, I do not know the exact criteria to reproduce this bug at 100%. I know that other devs do not encountered this issue when setting up their project. Perhaps this is due to the fact that my server locale is 'fr_FR' while a majority of devs use 'fr' here.