As you know OWA's UI or display language is determined by the Accept-Language HTTP header sent up by the browser on each request. This is controlled by a value set in Tools / Options / Languages... Now there are times when an OWA administrator might wish to override the language requested and return something else. One way to do this (if you have access to your front-end box) is to create an ISAPI filter which re-sets the Accept Language. This is described in the following KB (knowledge base) article.
(Thanks to Stuart Fox for the pointer)