The summary really says it all. I don't feel confident portraying a Heterolinguist from either end of things, PC or NPC. How do I have them speak? What does that even sound like? What, if anything, do they get from communicating with someone else?
They're definitely a challenge. On the plus side, there aren't many wrong ways to do it, as long as you're getting across the idea that there's some kind of communication difficulty. Heterolinguists are a) all about communication difficulties, and b) all doing it differently. So one of them might just talk kind of weird, another might remap words to different languages constantly (losing shades of meaning in the process), another might have developed some sort of musical or binary language that doesn't translate easily, another might have just excised all but the ten hundred words people say the most from their vocabulary.
Basically as long as you're having fun with it and your fellow players are amuse/frustrated more often than annoy/frustrated, you can't be doing it too wrong.
So, there's no way to do a heterolinguist perfectly - honestly there's no way to do any of the characters perfectly - but good enough isn't too hard.
My friend narfanator played a heterlinguist in the playtest campaign to generally good effect. In practice, remembering to keep to the bizarre speech patterns was hard and he slipped up frequently (especially when discussing strategy among the other players). That said, some of the more memorable quotes for me came from his character (especially “little bit of tinfoil” and “triangulate meaning”).
Start with what you want to say. Then, come up with a metaphor or weird reference to the thing you want to say. Cut extraneous words. Say what's left.
For example, let's say I'm buying donuts. I might say something like:
“I'd like two donuts, one chocolate and the other coconut.”
Let's heterloguist that:
“Dual toroids. Aztec bean butter and head-fruit.”
This brings us to the other bit of playing a heterolinguist: non-verbal communication. Tone of voice conveys a lot of information. You can express a statement as an order, a question, curiosity, friendship, or anything else with just tone of voice. So… exaggerate your tone, both to convey meaning, and when you feel like it to create more confusion.
If your thoughts start getting jumbled and you start using the wrong words for things or references that actually don't work, that's fine. Heterolinguists are by their nature largely incomprehensible, so “mistakes” are you doing it right. This can happen when you're trying to speak quickly and can't come up with metaphors fast enough, which is generally when your character is most stressed and wouldn't be able to counteract their own babble anyway.
So, that's the easy way to RP a heterolinguist. As for the deeper questions, heterolinguists do have language and can communicate, it's just that their language centers are somewhat scrambled. Heterolinguists don't want to deny themselves the ability to interact with everyone else, they just want barriers up to prevent metatech from working on them effectively. This makes communication difficult (so the more delicate metatech overlays can't make it through), not impossible. In real world terms, I think of them as having a mild case of Broca's aphasia (albeit bidirectional, and with an underlying structure to the jumbled-up language that exists, but isn't the same as normal communication).