theres another method thats very useful, especially if you're deploying multiple packs (or other resources). Simply copy/ftp the files to the Install folder. It contains a number of sub-folders, one for each type of uploadable resource. Copy your spanish language pack zipfile into the Language subfolder. You can now either wait for the ResourceInstaller scheduled task to automatically install the resources for you (the default is every 30 minutes), or kick off the task manually by using www.mysite.com/Install/Install.aspx?mode=InstallResources . Multiple files can be placed into each folder, and a status report will be generated at the end.
I've used this recently to install a dozen language packs, a few custom skins and some modules, all in 1 step, which hugely impressed the customer.
Cathal