Captions are stored in the web.config file.
All Captions begin with the "lbl" prefix. For Example:
<add key="lblWelcomeMsg" value="If you are an existing customer please log on here:"/>
To view which caption is against where in the portal turn on the "SetupCaptions" option by setting the value to "Y" (the default is "N")
<add key="SetupCaptions" value="Y"/>
Then when you view the portal you will see the "lbl" tag in brackets beside the caption.
When using the ASP.Net grid you can set which caption to use in the web.config file
<add key="captionLanguage" value="capt_us"/>