Don't feel too bad, I don't think the sentence is very well written. Do you understand the phrase "the situations outside the classroom for which learners are being prepared"? The second sentence speaks about using these situations as a yardstick for how a real language is used. To take something as a yardstick is an English expression that means to use something as a standard or to use something as an example. So, the second sentence could be rewritten like this: "If we take as an example of what is 'real' the situations outside the classroom for which learners are being prepared, this is undoubtedly the case." Does this help at all?

couldn't be better, thanks a lot.

These quotes do seem a bit out of context but I'll try my best. "The classroom is often called an artificial environment for learning and using a foreign language." - This sentence is trying to explain is that classrooms are an environment (environment means place) that is made (artificial means not real) for learning and using a foreign language. "If we take as our yardstick for what is 'real' the situations outside the classroom for which learners are being prepared, this is undoubtedly the case." - When we say "as our yardstick", what we means is that we are comparing two things. In this case, we are comparing the situations that are being "made" in the classroom to real situations that would happen outside of the class room. And I think what the end of the second sentence is saying is that if you teach someone to deal with a situation they might encounter outside the classroom then the "artificial environment" is truly doing what it supposed to be doing.