How can I uninstall this app?
Go to Settings > Security > Device Administrators and disable AcDisplay. You can now uninstall AcDisplay normally.

How to dismiss the permanent notifications of active mode or the lockscreen?
The notification is necessary to ensure AcDisplay isn't killed by the system. However, to hide it safely (sending test notification won't function anymore!) launch app info and disable showing notifications.

When will the secure lockscreen (pattern/PIN) be implemented?
It is not possible to create a custom, secure lockscreen for Android. Other apps that have this feature only pretend to be secure, but in the real world there's always a way to bypass them.

When will the double-tap to wake be implemented?
It is not possible to implement, cause it requires a kernel modification.

I can see a typo / mistake in the translation!
Thanks, you can easily help out with that. Join the app's page on crowdin and suggest a better translation.