Absolutely agree. I would never be able to take anybody seriously (in terms of hiring them) who can't spell or get basic grammar right in my native tongue, but

Absolutely agree. I would never be able to take anybody seriously (in terms of hiring them) who can't spell or get basic grammar right in my native tongue, but this tends to be quite common in written English. How much can one infer from the fact that a native speaker mixes up "they're" and "their"?