From the SF Core Rules (to remind us):
Telepathy
A creature with the telepathy special ability can mentally
communicate with any other creature within a certain range
(specified in the creature's description, though the range of
telepathy is usually 100 feet) that knows a language. It is
possible to address multiple creatures at once telepathically,
although maintaining a telepathic conversation with
more than one creature at a time is just as difficult as
simultaneously speaking and listening to multiple people at
the same time.
Limited Telepathy
Limited telepathy functions in the same manner as telepathy,
except that both the creature with the limited telepathy special
ability and the creature it is communicating with must have a
language in common.
From this, it appears Telepathy can be used to communicate via a two-way conversation with any being that understands a language. This is regardless of what each person involved can speak.
Limited Telepathy, however, requires both parties to have a common language.
In either case, it can be assumed by the writing that only one party in the conversation is required to have a form of telepathy.
Having said that, I'll be ruling as per Kvetchus has described until clarification!