Just to add a little to what utnik said, Scribus will only let you use font variants - bold, italic, etc. - that you have installed. If you don't have, for example, a Bangla Bold font installed then you won't be able to use one.
Other software, such as LibreOffice and GIMP, create bold and italic variants "on the fly" - if those variants are not installed - by messing around with the glyphs in the normal font.
For example, to create an italic variant they skew the glyphs in the normal font so they lean over. (It's probably more complicated than that but that's good enough for this explanation.) For most purposes - letters and artwork - this is okay but, as Scribus is more concerned with typography, the developers decided to not do this. If you look at a font that has a properly designed italic variant you will see that the glyphs are not merely skewed but they are designed differently.
See the two attached images. The first is the Elevation font "italicized" by LibreOffice. Note how the letters are simply skewed. The second is Times New Roman. Note the big differences in the "f" and "n", then see how the other letters are also quite different.
The upshot of all this is: If you need to use bold and/or italic characters you need to use a typeface that has bold/italic variants.