Both ways of forming the plural are commonly accepted and found in
most if not all dictionaries.  Even Google searches restricted to .uk
or .ca show more use of "formulas" than "formulae," although it's very
close with .uk.  A .au search does show more use of "formulae."  A
search of US domains shows a strong preference for "formulas."

If you believe that your dialect of English strongly prefers
"formulae," please take it up with the Gnumeric developers, and submit
a translation patch.  Strongly recommend closing as NOTABUG, although
I don't have permission to do so.

I am probably being rude in modifying a bug not owned by me, and I
also would not give my opinion if this is a bug or not.  In any case
this should be sent upstream and let the authors decide. ;)