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
    http://support.microsoft.com/default.aspx?scid=kb;en-us;q310599

    IIS 6.0: ISAPI Filters for Earlier Versions of IIS May Not Load
    http://support.microsoft.com/default.aspx?scid=kb;en-us;327611