If we enacted such a rule then people who dont speak fluent english would not be allowed on the instance.
If they dont speak english how did they read the rules and understand them or even know that we are a STEM instance for that matter. We are not a general purpose instance, we are not here to cater to everyone.
And if they do not live up to what they say? What if someone wants to come here with the sole intention of spamming advertisements in a foreign langauge and simply claims to want to come here for stem so they can get access? How would we prevent that after the fact in a way that is quick and before it has a chance to pollute the timeline for everyone.
You have to remember spam is a huge problem on the fediverse, just as much as any email server.