Yup, Colibri – it can be said that, as far as linguists are concerned, there is only ONE unassailable grammar rule:
If the recipient understands the message, then the message is grammatically perfect. No other requirements apply.
All the “rules” we learn in English class, and which are laid out in in Strunk and White? Those aren’t rules of good “grammar”, per se, but merely of “good style”.
Say someone speaking accented English watches Sylvester and Tweety cartoons with you. He cracks up and says “Ha ha! ‘I think I see puddy-tat!!’ Oh, is so funny, no?”
Now, do you understand what he is trying to communicate to you? Likely, yes – with the contextual cues, you’d have no problem wading through his stylistic eccentricities. Therefore, that speaker’s use of English is “gramatically” perfect.