To see where to translate I usually enable inline translation in
System > Configuration > Advanced > Developer > Translate inline
Enable it for the frontend of your store and save config. Now go to the frontend of your store and click on the little book icon that is next to the text that you wish to translate. A popup window will appear. There you will see the Scope for the translation, e.g. Mage_Core or Mage_CatalogSearch. This tells you in which .csv file to look for translation.
I prefer not to translate directly from the inline translation, since it will store the translation in the database (and not the .csv file). I only use it to find the right scope.
If you are using a custom theme then maybe there is a .csv file in the theme as well. Theme developers tend to place this file in
You may want to look there to see if you can find something like that. If this is the case then copy the file to local and create your own language folder with your translation file.