The sign-in button uses the variable <% signin %> to create the button text.
You can simply edit all occurrences of that replacing with the specific text you want, but I would not really recommend that approach.
You can define that variable overwriting the default by adding something like the following at the top of the page right before the <html> statement
<% SET signin = "sign in / s'enregistre" %>
My approach was to create a file called standard_vars.thtml that defines all the variables I want to set then include the following statement at the top of any page I want to use those variable. This makes it easy to change variable definitions.
<% PROCESS standard_vars.thtml %>
Here's an scrubbed example of what my standard_vars.thtml file looks like
<!--- General Variables --->
<% SET title = "My portal" %>
<% SET portal = "My portal" %>
<% SET color = "#FFFFFF" %>
<!--- PreAuth Variables --->
<% SET preAuthSNTitle = "Computer Systems Agreement" %>
<% SET preAuthSNProceed = "I Agree" %>
<!--- Login Variables --->
<% SET signin = "Login" %>
<% SET instructions = "For problems, please call the Service Desk" %>
