Was the sentence really wrong before? By making radical changes like this one you're invalidating the translations.

Hi Deniko, thanks for your comment. I notice you're not a native English speaker. The original "How expensive a piano is!" sounded very unnatural to me, which is why I changed it.

"What an expensive piano!" sounds good to me to. I will change it to that then. Thanks for the suggestion. I can't speak for the other languages, but the Esperanto looks singular to me.

Duplicates of this sentence have been deleted:
x #5661465

I can't judge how natural these English sentences are, but I'm quite sure only
"How expensive a piano is!" matches the German. Upon seeing the price of one piano, the speaker makes a statement about pianos in general.
"What an expensive piano!" refers to a specific piano.
Now, as both English sentences are linked to the German one, the only thing Manfredo, the owner of the German sentence, can do is to cut the link to "What an expensive piano!"
(which in reality he can't)

The best way to deal with this sort of situation in future is to create a new translation and then delete the unnatural English sentence.