Hi mtrowe,
How are you checking if the locale is getting set correctly? You should be able to see the different date, and number decimal locations by default, but the BIRT report content won't change unless you have assigned some localization to the report, and then created and deployed the correct properties files for each language.
As for the built-in viewer labels, these are controlled in a file called messages.properties, that you can also modify.... or create one for each language you want to support. There is more information on this at
Report viewer internalization