Translation:How far away are the beach and the sea?
I would never use the singular "is" when referring to "the beach and the sea", I would use "are". But then I've only been a native (British) English speaker for fifty-five years, so what do I know?
Quite right, and I would correct it if I could. However, the sentence is not showing up in the database. In what context did it come up? Were you using the web or the app, a single sentence or a multiple choice,... etc.
Note that the Welsh does correctly use the singular verb in that sentence, but English would use a plural verb
Edit - The sentence had been deleted at some point by the look of it. I have now put it back with the correct English translation. It may perhaps have been culled if its statistics had shown it as being too difficult for too many people. If that is the case then it will be culled again.