Monolingual

What attracts many users about macs is that almost everything works out of the box - but, once it's out the box,  you may not need everything that's installed on your system by default. Monolingual frees up disk space by letting you remove unused language packs from your hard-disk - most users never require Burmese or Catalan for day to day use.

Another wonderful simplification of installing programs is the universal binary - support for all system architectures put into a single file. However, OS X doesn't remove the architectures you don't need from the universal binary, so you often have unused PowerPC executables sitting round on your Intel machine, or vice versa. Monolingual lets you choose which architectures you want to keep, and it will get rid of the irrelevant ones, potentially saving hundreds of megabytes of disk space.

Developer's site