You are right that the key word here is "communicate," but unfortunately, because that word has no specially-chosen meaning in the RAW,
The answer varies depending on the creature summoned, and may not be covered clearly by RAW alone.
However, here are cases clearly covered by the RAW:
- Obviously, summoned creatures obey commands given in any language they can understand (listed in the Languages section of their stat blocks).
- Summoned creatures with Intelligence 2 or 1 can be commanded by using Handle Animal normally. The check is DC 25 (30 for non-animals), and requires a full-round action. (Because there is no evidence that summoned creatures are trained as an attack dog would be, you must use the "push" option from the skill.)
- Numerous special abilities and spells may be helpful, such as Speak with Animals.
The GM's intepretation of the RAW may permit other options:
- Commands may possibly be obeyed via Diplomacy, but only if the creature can "understand" you, has intelligence higher than 3, and "generally" only outside of combat. "Understand" and "generally" are also not clearly defined, so good luck.
- As a last resort, a Charisma (scroll to the bottom) ability check may be permitted when nothing else applies; discussions of RAW often miss the basics, but the basics are there for a reason: "You apply your character's Charisma modifier to . . . Checks that represent attempts to influence others."
I don't mention the Charisma check as a joke: Simple commands like "Attack that guy" can be implied well enough with two or three words and some pointing. Anybody can memorize two or three key phrases in several different languages. But that kind of thing is subject to GM interpretation.