Just in case you haven't already solved your issue, the answer is that
there is no way to change the language of an already installed Trial
Download. What you can do is to remove the Trial, and download / install
the correct language separately.
Also, if you don't need to test printing or document saving, the online
trial might be easier for you to try out:
http://office.microsoft.com/en-us/products/HA101687261033.aspx
Hope that helps,
Barb Way
Product Support - Visio
Microsoft Corporation
This posting is provided "AS IS" with no warranties, and confers no rights.