Sometimes when a phrase gets translated it takes a longer or shorter amount of time to say in the other language so you have to add to it or truncate it. Otherwise the mouth moves onscreen but there's no voice or there's a voice but the mouth isn't moving. Sometimes they switch things up to try and have the word being said sound similar to the original so that the phonemes look the same, too. I can't imagine the amount of work Sherlock's doubles have to do get their timing right with his rants in each language.