Unfortunately as you note Marcel localisation doesn't go that far but you can make your own work around.
The default buttons used in the templates are held in their own folders in the module you are making. Look at the folder named "1" and say "nav_008_next. This is the default next button and contains four image states for the button.
Find this and using a paint program change the text on the default buttons.
As you'll want to reuse these keep a copy outside of courselab and just drop them into your next module.
It's not a perfect solution but it works