I've used Monolingual a couple of times on my iBook with no ill effects. You just have to pay attention - US English (along with almost every other localization) is selected for removal by default - you have to uncheck your preferred localization to KEEP it (and you need to keep the one called "English" as well, as the US English localizations are based on that one). A quick look at the Readme file provides these important instructions (in bold print, no less).