You'll have to reinstall any apps that you need the language for. Be aware that extreme caution should be exercised when using apps like Monolingual, as deleting certain languages or architectures can create significant problems. At best, deleting these things may buy you 1-2GB of drive space, but will not improve your system's performance. If you're not certain about what you're doing, don't delete anything. The small drive space savings are not worth the risk of potential problems.