"We are flying up too!"
Translation:Felrepülünk mi is!
Thanks! That makes me think having "mi is" first would be a closer fit to the original. This English sentence seems much more likely to be about adding "we" to the list of people who are flying up than adding "flying up" to the list of things we are doing.
Were the verb something a bit more everyday, I suppose it'd be more ambiguous: "we're getting our car washed, too" could easily follow "We're going to the grocery store" or "the neighbors are getting a car wash."
In this one, logically, you probably do need it. The point of the sentence is almost certainly the "We too!" part of it. And so you need mi is in there.
Felrepülünk is without the mi in it is a possible sentence but it means "(In addition to something else we're doing) we are flying up too."