Verbindung is made by one small studio, so support is a direct email rather than a helpdesk. Messages reach the person who wrote the code. Expect a reply within a few days, slower at weekends.
Three things make a bug fixable on the first reply:
On your device. Review history, streak and any entries you edit never leave the phone. There is no account and no server. See the privacy policy for detail.
Because progress is local, it moves only if Android's own system backup carried it across. That backup belongs to you and to Google, so we cannot restore it from our side. If the backup did not include the app, the new install starts fresh.
Subscriptions are tied to the Google account that bought them, not to the device. Sign in with the same account and the app should recognise it. If it still shows ads after that, email us with the order number from your Play receipt.
Send it. Regional usage varies and the set is large enough that mistakes are possible. Include the entry and what you would expect instead. Corrections that hold up go into the next release.
Verbindung uses the text-to-speech engine installed on your device, so German has to be available there. On most phones that is Settings, then Accessibility, then Text-to-speech, where the German voice data can be downloaded. Also check the mute switch inside the app.
Offline, yes. Ads need a connection, so studying in aeroplane mode gives you the full app without them. That is not a workaround we mind you knowing about.
Feature requests are welcome and get read. The app is deliberately narrow, so the answer is often no, but a clear description of what you were trying to do is useful either way.