How can I localized the text content of the "10% Loaded" (when the form is first loading) and "Please Wait" (when saving/submitting/etc.) spinners?
This is done through the Localizable Strings object in Nuts & Bolts -. Form Options
Glenn is correct as to where you localize the strings.
However, I believe this is currently not working due to a bug https://support.avoka.com/jira/browse/TPD-3793
This will be fixed in the upcoming 4.4 sp1, a patch is also available currently on test with this fix in it