You might want to look into localization: http://livedocs.adobe.com/flex/3/html/help.html?content=l10n_1.html
It is theoretically possible to set the values in CSS, but it would require creating a new set of components that added a text CSS element. I would advise against it. If you really need to load the labels at runtime, I would suggest using an XML file. CSS isn't really suited for that kind of task.