hi Adam,
wouldn't it be better you to tell Jolly this is an issue with the WCEM? - do you find 'reasonable' not being able to switch languages between requests?
@Jolly: keep "com.sap.wec.localeDeterminationMode" as request, dispatch an event when the user "switch" the language (fixing SAP source) and intercept this event on your Module(s) Event Handler (in whatever module you need to re-create the Business Objects) and start re-building the objects with a proper locale.
Hope it helps,
D.