There seems to be an error in translation on the course “German 4” from french. The sentence is “sei ruhig” which is translated as “être sage” (“to be wise”). The verb tense isn’t even the same: “sei” is 2nd person imperative and “être” is infinitive. And as to the Larousse (french bilingual dictionary), “ruhig” only translates as “câlme” and not “sage”. So the french translation for “sei ruhig” should be “sois câlme”.
The forum thread dedicated to this course can be found here. I suggest that you repost your query there.
As for the error itself, I’d rather translate “sei ruhig!” (or “sei still!” which would be the more appropriate way to say that) with “tais-toi !”. Also to be noted: in both languages the exclamation mark should be there in order to illustrate the imperative.
Edit: rethinking this, I suppose “sois calme” probably is the best translation for “sei ruhig”. It’s probably that I (personally!) am not (or no longer) used to saying that as my kids have been grown up for a while now already.