Hi ....
I`m a brand new member living i Denmark ....

My question is : is it possible to make UG write Bb instead of A# ...and so on ?

Kindly ...Carl Sandhoj
In flat keys, Bb usually makes more sense. Would probably work best as a sharp/flat toggle.

Come to the Site Suggestion sub-forum under UG Feedback; this is an off-topic forum
Both are needed but I understand where you're coming from. Many contributors, including some high ranking ones, ignore keys and misname chords in this way. You would think it would be a fairly easy thing to correct.