I think it is broken on v5.0.3. I have only the default install and no extra applications installed.
It works fine in the ACP, but once I go to the live forum it doesn't work. It just shows for example forums as: #VLE#forums#!# or the home button as #VLE#home#!#

The navigation buttons can be translated but that is pretty much it. I think in invision 4 it was simple to translate everything.

We have open bug reports for this one :)