Polaris Office cannot set Date Format to UK
Yes. If an app doesn't work the way you want, or you can't work out how to make it do what you want, it's always appropriate to contact the developer.

You can do this via Google Play: every app has to have an email address or phone number for support, which you can find on that app's page. Many apps also have a "support" or "feedback" option inside the app too.

Even if the answer is that there's no way to do this and they don't intend to add it, at least you know that from the horse's mouth, and of course if they get enough requests they may change their mind and add it. And going the other way, even if there was already a really simple way to do it, and they just tell you how to do it, it lets them know that they need to make it easier to find in future.