Every day people who have little grasp of English attempt to answer questions in their own uniquely inept way; they have no conception of which words work with which other words and they rarely provide sources for their answers. I believe this can be mitigated with a mandatory English quiz given when signing up for the site. It'll lessen the strain on the community to delete these posts. Why hasn't it been done yet?
A strain on the community? How hard is a downvote on a poorly written question?
My account is two years old, but I started off few months ago.
I'm not a native English speaker, and I have made mistakes a lot.
If such a quiz was mandatory, I wouldn't even have joined at all.
To answer your question of why it hasn't been done yet: It had not been requested before.
The number of downvotes here indicates that the community does not support this plan. Therefore, it is safe to conclude that it won't be implemented.
I can understand why you've proposed "taking mandatory English quiz". Yes, it is frustrating to see all those low-quality posts and find that we are not doing enough to prevent or slow down the influx of low-quality posts. My question is "Is there anything we can do?"
One user once proposed all questions be put on hold automatically and users vote to open an on-topic question among them. This suggestion illustrates there have been so many off-topic and low-quality questions and answers posted and deleted on ELU.
ELU is open to everybody in the world. Even a user who doesn't bother to register their user name can post a question and answer (except to closed and protected questions) and edit any post they want.
We can't stop new users from asking, answering a question and editing a post. We need to
Post a comment for clarification, additional information, request to show research.
Flag a question if you don't have 3,000 reputation points needed to vote to put a question on hold.
Downvote a post which is not useful and doesn't show research efforts.
Monitor suggested edits, especially those submitted by anonymous users, and review first posts and late answers carefully.
There are mechanisms we can use to deal with low-quality posts. I am one of the most aggressive users who are employing them to clean up the site. Even though asking new users to take mandatory English quiz might help reduce the number of low-quality posts, it is not an ideal solution considering the manpower and time it will take.