[whswhs]

I want to say, first off, that there are two different restrictions here. There are physical restrictions, such as not having sophisticated enough vocal organs for articulate speech. That's -15 points and trades off for the Signals advantage from Template Toolkit 2, which lets you have a "language" based on skin color changes, electric pulses, or some other exotic channel. But then there's not having the cognitive ability to use a sophisticated language, which I think is what's involved here.

I think basically I would go with malloyd's idea that this is a zero point feature: a taboo trait. But bear in mind that if you're a normal character with IQ 6 or better, you get Native use of one language for free. So if you're from, say, a Spanish speaking country, and you can't manage better than Broken Spanish, that's a -4 disadvantage (or -5 if you can't read or write at all). I think you should take that disadvantage, and THEN take the Taboo Trait to prevent your learning either your native language or any other language above Broken.