"If it isn't Mr. Smith?" *is* an expression That sentence gets said when Mr. Smith walks into a room. (or when you walk into a room where Mr. Smith already is). It is filler. It does not really mean anything, other than acknowledging that Mr. Smith is physically present; there may be meaning in the tone (it can be said with a surprised tone or a relieved tone or an angry tone or some other tone - depending on how you feel about Mr. Smith). It will also immediately be followed by some other sentence like "Hi." or "How are you?" or an introduction between Mr. Smith and the other people in the room. I have no idea how you could translate that (I might use the same translation you would for "Mr. Smith is here," but it isn't quite the same)

No mystery here, just translate "If it's not..." ("if he's not"?) and add Mr Smith to the phrase. It's not an idiom, but maybe our impersonal "it" is included just to mislead you.

"Smith" is the most common English surname (last name) so it is often used as an alias when somebody doesn't want to provide his real name.