Hi, thanks for your answer! I’ve tried removing the Scale Box, but now longer texts always overflow the box unless I reduce the font size, which is not desirable for shorter texts because it leaves too much unused empty space.
Besides, I can’t predict how much longer or shorter these texts will be in different languages, that’s why I was counting on a Scale Box to reduce the size if they were too long to fit at the default size.
As for the performance cost of applying manual wrapping that you mention, rather than checking the width for each character appended I was planning on setting the full text and then calculating and setting the Wrap Text At Value. Something similar to what this person was trying to do: UMG: Scale Box and Wrap Text - #2 by SolidSk Do you think that would be as expensive? 
Thanks again!