Due to a recent influx of low-quality questions on this site I think we should have a sandbox on this site.
Should we create a sandbox in this meta site, or is the stuff we have here enough?
We don't need this.
Game Development Stack Exchange is a question and answer site for professional and independent game developers. It's built and run by you as part of the Stack Exchange network of Q&A sites. With your help, we're working together to build a library of detailed answers to every question about game development. (Source)
StackExchange sites already have all (well, most likely 'most of') the tools required for moderation by the community. This includes
The site is made in such a way that it will allow users to fail a bit and learn from it before preventing them from posting trash on the site.
If you're really worried about if a question fits well on the main site, either ask it there and wait for feedback and learn from it, or ask in chat and see the reaction of the users there.
Finally, from my own experience, questions asked in meta that should have been asked on the site instead are generally from users trying to evade a question ban on the main site. We won't open the door to it.
In part, this is what the review queue is for. Special attention is called out toward helping new users.
If we had as much traffic as Stack Overflow, we might also have a triage, but that's not yet needed here.
In my experience, the current system is already really effective at actual spam (ads & self promotion). After that, there are reputation & badge incentives to encourage everyone to edit posts.
Keep in mind that most users are not intentionally making bad posts. Mobiles aren't the most elegant way for submitting questions, but that's all some users have to work with. English is not a universal language. even in places where it is a default language, some use it better than others.
The barrier you propose strikes me as being at odds with the 'be friendly' attitude & code of conduct, specifically:
building a community where all people feel welcome and can participate, regardless of expertise
Asking good questions, adapting to our norms & observing our protocols are skills that improve with practice & support from other community members.