This page covers a few common issues known with watch apps. If this page doesn't answer your questions, you can contact me at email@example.com
First, check whether Bluetooth is still active. If it is, please try to restart your iPhone and your Watch. Sometimes this is needed to get the connection running again.
If Threema confirms your scan but the app on the Watch is still showing the QR code after 10 seconds, please check whether there is a red exclamation mark ([!]) next to the newly created session in the Threema list. If that is the case, try to perform the scan again. Threema Web for iOS is quite a new technology so sometimes you might run into issues setting it up.
Apple Watch uses the language that has previously been used on your iPhone keyboard. Add the desired language to your keyboard list, type a few characters with this language and your Watch will most likely start to understand you :)