Before you start translating, you should do some checks to make sure that you are ready to go.
Check the Current translation status pages and see if your language is listed. If it isn’t, then the team really needs you to help out. If your language is already listed, you should try to work together with the current translator of that language, and split up the work so you are reducing the workload, and increasing the quality of the translation. Many translators are happy to share the work or even appreciate people discussing translations.
In the ‘‘.po” file of your language there should be a string that tells you the last translator. You can look up the po file in svn for instance, and view the contents as a webpage. Here is an example link, that shows you the french translation of the xfce4-panel translation .po file: http://svn.xfce.org/svn/xfce/xfce4-panel/trunk/po/fr.po. In this file, you can read that the “Last-Translator” was some person (Maximilian Schleiss). Contact him to see if he needs help if you want to update the French translation of the panel.
Sometimes people are already working on translations for your language, but they haven’t submitted or committed their translation. It helps to announce that you are working on something before you are actually doing it, and more people might help you out and reduce the workload.
Visit the language_maintainers and language_maintainers:talk pages and look around. You should also consider subscribing to the i18n mailing list http://foo-projects.org/mailman/listinfo/xfce-i18n and tell your intentions there by starting the subject with your language code like [fr] for French or [pt_BR] for the Brazilian Portuguese.