For documentation on hyphenation, read http://documentation.scribus.net/index.php/Hyphenation
Issues on debian
Debian's scribus 1.4.4 users regularly have issues with hyphenation in various langages.
Gabo describes how he sorted this issue (for spanish language) :
Under the preferences is the option to download some dictionaries.
I have to move the downloaded dict to /usr/share/scribus/dicts and /usr/share/hunspell (not sure which one is the correct location)Also the /usr/share/hunspell was renamed to hyph_es_ES.dic just like the existing hyph_en_US.dic
Fine tuning of hyphenation
If a line after the last line that features a hyphen has too large word spaces, you can adjust the hyphenation manually with soft hyphens (also called smart hyphens). You can use the menu entry insert > character > smart hyphen or its ctrl + shift + - shortcut. Or you can remove hyphenation locally. Select the text for dehyphenation and run Extras > Dehyphenate.
Also, If you want to squeeze some text, select it and set horizontal scale to 94-98%. This works with most text-sized fonts. A difference between normal and condensed script cant't be seen.
With 1.3.4 you can create manual line breaks, so that if the word starts with a soft hyphen, it does not hyphenate.
Further reading about Hyphenation in combination with Full/Forced Justification.