Pls change the language in WordPress to your language, Spanish, follow instructions provided in http://codex.wordpress.org/Installing_WordPress_in_Your_Language. Edit the .po file using editor like the PoEdit – http://www.poedit.net/. You should be able to locate strings like leave a comment, search, post a comment etc. and input the corresponding words in Spanish. Read instructions above to know how to use the .mo file generated by PoEdit. Let me know if you still have trouble.