I successfully used Monolingual on Tiger last year with my new MBP. I only removed languages -- and I was very careful about selecting what was removed. The Monolingual defaults looked like they would have removed English and made my Mac unusable -- I don't understand its defaults, but I think you need carefully review them as they can't be trusted.
I've not tried it since upgrading to Leopard; I didn't know if there were any incompatibilities introduced since then, as Monolingual hasn't been updated specifically for 10.5.