How do I force OWA to only render the web page in Engish?

There is no supported way to do this.  OWA uses the IE language settings to decide which language to display. Remember that OWA is a web application and goes off of browser settings just like most every other web site out there.  During the intial log-in to OWA you can set the language to use in OWA.  You can also override the language setting once logged-into OWA.  However, there is no supported way of forcing Englishe or any other language to be a speicific language other than to change IE (or other browser) language settings.

Below is an unsupported work-around for this issue.   Such code would be not supported or advised to use with OWA – so its a “use at your own risk” deal.  While there is support for writing ISAPI extensions and filters, there is no developer support for getting such a filter to work with OWA traffic.  ISAPI filters/extensions are notorious for messing-up OWA.  If such a filter were to be used and you need to call in for support, you may be asked to remove such a filter for troubleshooting.

    How to hard code the language of OWA interface;en-us;q310599

    IIS 6.0: ISAPI Filters for Earlier Versions of IIS May Not Load;en-us;327611