Windows and Mac os
Just go to your installation folder
ex.: c:\Program Files\TechSmith\Camtasia 9\
and rename the folder of the current language into anything else or force the folder to be non usable/writeable.
For example, with german:
c:\Program Files\TechSmith\Camtasia 9\de-DE
The best solution is to download the trial from the official website that is in English language and when you can choose the language while installing