Hi Sophie,
I was going to close the bug but when I submitted Pedro had done it at the same time, so I kept only his changes and cancelled mine:
Bug confirmed under Windows XP x86 using LO 4.2.1.1 with Portuguese UI
The text with Tahoma font is indeed tighter than when the English UI is used. Please notice that the odd formatted text is set to Character Spacing: Very Tight
However selecting the text and clicking on Very Tight (i.e. reapplying the style) with the English UI produces the same result on the font.
However the black bars only show up with the Portuguese UI
Please let me know if that solved the problem 
Glaucia Cintra