Communication. Either your DM is incredibly generous and lets you learn exotic languages (mine doesn't as part of character creation) or you have to cast tongues for each elemental. Of course, if you just want the elementals to be friendly towards you and defend themselves when attacked, you don't have to worry about being able to talk to them.
Or, you could cast tongues on yourself which will let you be understood by "any creature that knows at least one language."
Though I hadn't thought of how the language barrier could inconvenience a summoner.