Although this plugin works fine, I'm afraid it fails when running alongside qTranslate.
The cashed widgets versions are served but they might be in a different language than the one the visitor has selected.
I can imagine it might be too complicated or too much work (core recoding?) to make Widget Cache take either the language or URL (where a language string will be used) into account when storing and serving from cache, but it would really be great addition to the project...
In any case, thanks for a great plugin :)