I have telepathy as the MM1 says:
Telepathy (Su)
A creature with this ability can communicate telepathically with any other creature within a certain range (specified in the creature’s entry, usually 100 feet) that has 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. Some creatures (such as the Pseudodragon) have a limited form of telepathy, while others (such as the Formian Queen) have a more powerful form of the ability.
Another player said that I cannot speak to someone unless I have the same language as them, according to the "language rules". Is this true?
Best Answer
The other player is incorrect.
If you have telepathy, you can communicate with any creature that knows any language, regardless of whether or not you speak that language. As you quoted,
That is the only requirement: a language. Not any particular one, not one shared between you, just capable of language in general. It’s unclear whether it works on creatures that understand but cannot speak any languages, but that’s a corner-case not relevant to the usual uses of telepathy.