Translator Microbes are a species of extraterrestrial microorganisms which colonize the "base of the brain" of many sapient races and allow them to understand spoken alien languages instantly, thus bypassing the need to actually learn the language.
Biology and cultural use
Strictly speaking, only the listener requires having the microbes to understand what is being said to them. But the microbes will not allow them to make themselves understood by those who don't have them. As such, two-way communication between aliens will require that both parties have microbes (or be otherwise capable of understanding the other's language).
Species known to inject themselves with translator microbes notably include the Sebaceans, Hynerians, Luxans and Delvians, among many others. A comment from Dominar Rygel XVI suggests that it's customary, at least for Hynerians, to be injected at birth. However, it's also possible that the microbes might be naturally transferred from mother to offspring, given that some non-spacefaring races which are descendant from spacefaring ones appear to still have them, such as the Acquarans.
Some species, such as the Pilots and Diagnosans, have languages that are too complex to be properly translated, and thus need to speak in a simplified fashion in order to be understood by aliens. Not all species are compatible with translator microbes. The Kalish notably can't host them, and thus need to learn alien languages by themselves.
It's still possible to teach and to learn alien languages while having the microbes, which probably facilitates the process. In the case of written languages, this learning process is indeed required, as the microbes won't translate written text automatically.
Living ships such as Leviathans, while not capable to speak to aliens under normal circumstances, have Diagnostic Repair Drones that are equipped with translator microbes and can inject them into alien visitors to allow them to communicate with the Leviathan's crew.