Tweet of the Day: Writing Excuses 8.23: Microcasting
If you need a quick explanation why everybody, be they aliens, angels or ancient Romans out of time, speak perfect English (or the story chosen language) then this is it. It could be a device, spell or effect of the environment. Often the trope avoids rare words, idioms, or cultural references, but only if the author wants to make a play on words. Other limitations include new or very old languages, or creatures that communicate in strange ways.
This trope is common in speculative fiction because not everybody is a gifted linguist like Tolkien that can create entire languages out of thin air. The author most also remember when the microbes are not in effect, such when a scene aliens speak among themselves, sans the main character presence. If they still speak in the works chosen language then it is Translation Convention.