Quit a lot of the IDE is already translated. You can change the language in Tools, Environment Options, Desktop and choose the appropiate language.
Restart the IDE. Missing translation are shown as blanks.
People working (or having worked) on the Spanish translations are Luis Hilario and Raul Moratalla. You can probably connect them via the lazarus mailing list (you need to subscribe to be able to post).
To make a translation, you have to edit the various *.mo.es files, you find in the languages subdirectories in the lazarus source tree, e.g. languages/objinspstrconsts.es.mo.
HTH,
Vincent.