"Czech piano's have a beautiful sound."
Translation:České klavíry mají nádherný zvuk.
there should be no apostrophe in piano's in this sentence. It is simple plural, not possessive, and not short for IS (THE PIANO'S OUT OF TUNE is fine, as is THE PIANO'S LEGS NEED REPAIRING, but not PIANO'S HAVE A BEAUTIFUL SOUND)
Sorry if that's pedantic, but that's English grammar for you.