Downplayed: The aliens speak simple, heavily accented English and frequently consult their equivalent of a phrase book, often having to just use their own words for things and hope Bob can tell by context.
The aliens have put a lot of effort into learning the Earthlings' language, but it never occurred to them that there is more than one language on Earth. When they meet Bob it turns out Bob only speaks English and the aliens only speak Chinese.
Zig Zagged: The aliens speak English perfectly well at first, only later they say they have trouble with certain phrases, even though they've been speaking fluently and said they have a total mastery of the language. Bob offers to converse in the aliens language, but no one on earth can learn it, so the aliens go back to speaking fluent English and the matter is never raised again.
Aliens arrive on Earth and meet Bob. They realize that they forgot to learn any of the native languages of the planet, leaving both the aliens and Bob flummoxed about what to do now.
There are no aliens in the work.
"We need the aliens to speak English. It'll show how intelligent they are."
Alternatively: "In order to move the plot along, we need the aliens and the Earthlings to communicate with each other without any language barriers. Make the aliens speak English so our English speaking audience can understand them as well."
Lampshaded: "They have English classes on your planet?"
Invoked: Before arriving on Earth, the aliens have a quick review of the English language.
Exploited: Bob overhears the aliens' conversation which wasn't meant for him, and has no trouble understanding it because it is in perfect English.
The aliens do not bother to learn English or any other language before visiting Earth.
The aliens are confused as to the correct language to use when visiting Earth. They go through several different obscure languages to no avail.
Discussed: "Just wait: once the aliens arrive, they'll speak English fluently."
Conversed: "Are all aliens required to take English classes before visiting Earth?"
Implied: When Bob tells Alice and Charles about his encounter with the aliens, he tells them what they said to him - implying that he understood them.
Deconstructed: Aliens arrive on Earth and meet Bob. They speak to him in English. Bob notices that all the alien races manage to speak English, and tries to find out why it is so.
Reconstructed: Aliens arrive on Earth and meet Bob. They speak to him in English. Bob is so amazed by this that he accepts the aliens as members of a peaceful race and is made their spokesperson on Earth. Diplomatic relations are forged between the aliens and Earth, marking the First Contact in human history.
Played For Laughs: The aliens speak English with an outrageous Cockney accent that most people cannot understand.
Played For Drama: Aliens arrive on Earth and meet Bob. They speak to him in English. This frightens Bob, who thinks they only learned English as a means of helping to conquer the planet. Earth attacks the aliens, who are confused that their sign of cultural understanding was taken as a threat. The aliens either leave or retaliate in defense, possibly instigating World War III.
Plotted A Good Waste: "Don't bother coming up with a language for the aliens. Just have them speak English like in so many other works of fiction."