Basically, the microbes colonize the recipient's brain and allows them to understand alien speech. Only the one listening needs the microbes, and it translates the language they hear, not the language they speak. So if person A has microbes and person B does not, Person A can understand person B, but person B cannot understand person A.
There are canonical instances where language was too complex for the microbes unless it was simplified.
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http://farscape.wikia.com/wiki/Translator_microbe
Basically, the microbes colonize the recipient's brain and allows them to understand alien speech. Only the one listening needs the microbes, and it translates the language they hear, not the language they speak. So if person A has microbes and person B does not, Person A can understand person B, but person B cannot understand person A.
There are canonical instances where language was too complex for the microbes unless it was simplified.