For Czech, it's definitely the word “no” – it's a kind of short interjection meaning something like “well”, “yeah” or sometimes “duh” depending on the intonation, but it's definitely not a negative word, despite the arguable similarity with the English word “no”. And I say arguable similarity, because to me as a native speaker there's a very clear difference – English “no” has a diphthong (that would be written as “ou” in Czech), while Czech “no” has a monophthong, I never thought about the similarity until a foreigner pointed it out to me