From version 0.8.0, Stellarium fully supports translation into other languages. The translation is performed using gettext. All the strings used in the program are translated and saved in special .po files. These .po files can be edited using a text editor or special programs such as Kbabel or POedit.
Once your .po file is ready, you can compile it into a ".mo" file and test it in the program to check that everything is correct. For example, if you are French and work on "fr.po", save your compiled file "fr.mo" as "stellarium.mo" and copy it into the directory "..../stellarium/data/locale/fr/LC_MESSAGES/".
Re-run stellarium, select your language, and there you are!
An easier way (but less efficient) is to use the Rosetta facility which allows to translate Stellarium online, via a website:
If you work with an external editor like POedit or Kbabel, don't forget to remain synchronized with the online Rosetta version of your .po file. If not you may destroy the modifcations made by other contributors. To keep in sync, always restart work from the latest online version of the .po file, and always upload your modifications when you stop working on it. If you are not sure, it is safer to work only online in Rosetta.