What you say about children of Japanese people living outside Japan - yes, I've seen exactly that. Every day, in fact.
As for my English example.. it's grammatically a mess, the word order is wrong etc, but if you hear that sentence, how would you interpret it? There's really only one possible way of understanding it. I hear a lot of that kind of English, though not usually as messed up as that.. but sometimes pretty close. And it's still comprehensible, as in usually not ambiguous in practice. When comparing languages like this, of course it's about a low level understanding of the language. With English you can go a long way with very little knowledge of the language, with, for example, Italian you at least have to learn the verb forms because they substitute for pronouns as well.
As for my English example.. it's grammatically a mess, the word order is wrong etc, but if you hear that sentence, how would you interpret it? There's really only one possible way of understanding it. I hear a lot of that kind of English, though not usually as messed up as that.. but sometimes pretty close. And it's still comprehensible, as in usually not ambiguous in practice. When comparing languages like this, of course it's about a low level understanding of the language. With English you can go a long way with very little knowledge of the language, with, for example, Italian you at least have to learn the verb forms because they substitute for pronouns as well.