To set spanish like default language you must click in the "webmin" link just a side of the "Virtualmin" title (top on the left pannel) see something like this:
^ Virtualmin # Webmin <<--- must click here
Then you get into the left panel a list of options, click on "webmin", several links will appear. Click on "Change Language and Theme"
On the right panel locate the "Webmin UI language" option and cick on the "Personal Choice" radio button and from the option list select "Spanish (ES)" and click on the "Make Changes" Button
On the left panel click on "Usermin Configuration" and click on the flags icon (Languages icon) and locate the "Mostrar en Idioma" option and select "Spanish (ES)" then click on "Cambiar Idioma" Button.
Again on the left panel make click on "Webmin Configuration" again click on the flags icon (Language icon), select "Spanish (ES)" and make click on the "Cambiar Idioma" Button.
Now every time Virtualmin (partial) will show in spanish language, the translation its not finished yet so some text could be in English.