I don't think it's good idea to make such limitation because theymos already stated it's not need to be perfect English, I think 70% score is considered well written since they need to look at the grammar, not misspelled or using an incorrect words. I'd say it's better if they check the misspelled and try to use more appropriate words, because some people blatantly using google translate from their local language to English and that's make the sentence is odds.
I'm example of not a perfect English and I try my best to express my own opinion, I think even my grammar is bad, you can still understand what I'm trying to say it's? At least it doesn't make you confuse.
Just let you know, everyday when I have free time I always try to learn English to write more correctly, but I'm not sure where's the problem since I still couldn't write like a native, perhaps it take times.
Languages
If you are fluent in any language other than English, then it is highly encouraged for you to post in your local board. These boards often have tight-knit communities which will be able to help you, and in some ways you might be at an advantage compared to English-only posters.
In the English sections, only English is allowed. It is not necessary to speak perfect English, though you should be understandable. Try your best. If you're unsure whether your English is good enough, ask in your local board or in the Beginners & Help section
However this kind idea is similar like newbie jail where theymos disagree about it and would damage the forum nature.
Limiting newbie participation is very harmful for a community. Newbie jail will never return: I consider the newbie-jail period to have been extremely damaging to the forum. When barriers to participation are too high, then the best people often just won't go to the trouble of joining, and the people who are willing to jump through the hoops are often people who aren't good for the community: people with nothing better to do, scammers, get-rick-quickers, etc. Having a permanent newbie jail policy would improve things a lot in the short-term, but would end up being a fatal poison to the community.