Some signs are in Belorussian, some are in Russian, but since the languages are very similar you shouldn't have problems understanding them. Everyone can speak Russian, even if Belarussian is their native language, but for majority native language is Russian.
About safety, I have to confirm that - from my experience the police are very friendly and uncorrupt compairing to Russia. Modern Belorussia reminds me USSR how it was in early 80, the good side of it.