How can I launch Scribus in my language?

From the command line you can run the following commands:
 * scribus -la
 * scribus --langs-available

This will give you a list of the language abbreviations and associated languages currently installed with Scribus.

You may then start Scribus using 5 methods to select a language, in the following order of predecence:
 * 1) Run either of the following commands, where the language_abbreviation is from the list you generated above.
 * 2) *scribus -l language_abbreviation
 * 3) *scribus --lang language_abbreviation
 * 4) Set the LC_ALL environment variable and run Scribus
 * 5) Set the LC_MESSAGES environment variable and run Scribus
 * 6) Set the LANG environment variable and run Scribus
 * 7) Let Scribus detect the Qt locale and run Scribus

In the case where your system has 2 or 3 of the above environment variables defined Scribus will use the first in the list above. Using the command line option of -l/--lang overrides the rest of the choices.

Scribus 1.3 also has an option in user preferences which falls after the command line option in precedence.