To rename Validate Button, you go in Admin-> Languages.
After that, under "Static Resources" you click on the edit button of your choice (under "Site" or under "Host").
Then, you expend "Aricie.SimpleSignIn" in your Local Resources. In expending "App_LocalResources", you click on Formulaire.ascx and modify the value "Resource Name: cmdSubmit.Text".
However, if you have several languages on your web site, this operations will have to do for each language you want to configure.